What does a nine digit zip display in the merge without the switch?
The conditional field will only produce the require result if it produces
nine digits
123456789
if it produces
000000000
00000
12345-1234
then it is not going to work as it stands. It will probably work best if the
column is formatted as simple numbers.

If the data is correctly displayed in Excel then From the Tools menu in
Word, select Options and then go to the General tab and check the box
against the "Confirm conversions at open" item.  Then when you attach the
data source to the mail merge main document, you will be given the option of
using the DDE method of connection which should read the data as you have it
formatted in the table.

I certainly wish it were simpler, but the "how" boils down to "different
software has different target groups, has been designed and implemented by
different people at different times, sometimes with no good standards to
work to"

I've posted a shortened version of this message in reply to your most recent
posts...

However, you may find the following helpful (and Graham - if you think it is
worth incorporating any of this in your web pages, please do so: I'll
probably also put it into my t0003 page about Excel-related problems.
However, the following results from a fairly quick look, not years of
experience)

Generally speaking, switching to the DDE approach that Graham has mentioned
will sort out a lot of problems using Excel data sources. It doesn't work in
all cases (e.g. if you do not actually have Excel on your system, the data
is not in the first sheet of the workbook, or you need to use "unusual"
Unicode characters) but in many cases there is a simple way of dealing with
that (e.g. if the sheet isn't the first in the book, copy the workbook and
delete all the sheets before the one you want).

1. In Excel, you can enter, store and format, ZIP codes in a number of
different ways, including:

a. If you enter 5-digit ZIPs by entering the 5 digits, Excel will save the
data as a number with "General" formatting and display the number, right
justified, with no leading zeroes. If it is possible to have ZIP codes with
leading zeroes, you can ensure that you always see all 5 digits by applying
the ZIP format, which is a numeric format: 00000.

b. If you enter 9-digit ZIPs by entering the 9 digits without the hyphen,
Excel will save the data as a number with "General" formatting and display
the number, left justified, with no leading zeroes. If you want to display
them with the hyphen, you can apply the ZIP+4 format which is a numeric
format: 00000\-0000 (i.e. applying the format does not change the stored
number into text)

c. If you enter 9-digit ZIPs by entering 5 digits, a hyphen, then 4 digits,
Excel will save the data as text with "General" formatting and display what
you entered, right justified.

d. If you format your cells as text, what happens depends on when and how
you do it:
   i) Once you have entered a number in a cell that is not formatted as
text, Excel stores it as a number, even if you apply Text formatting to the
cell.
  ii) If you format a cell as text, /then/ enter a number, Excel stores the
number as text (actually, I am by no means sure that this is always the
case). But it also flags a warning using one of its red(?) corner markers.
 iii) If you already have numbers in cells in a column and you want to turn
them into text format, you can do it, but not by applying text formatting -
you can
      - select the column
      - select the Data|Text to Columns... menu option
      - click Next through the wizard until you  reach Step 3 of 3, then
select Text as the Column Data Format.

As you can imagine, it is not all that easy to tell the difference between
the different things merely by looking at them. Typically, the 9-digit
number in (b) will be right-aligned even when it is displayed with the "-",
and the text in (c) will be left-aligned, as long as you have not explicitly
aligned the result.

Also, applying either of the ZIP or ZIP+4 formats to the entire column is
not much help because
e. if you apply the ZIP format to 9-digit ZIPs, you just see a 9-digit
number with no hyphen
f. if you apply the ZIP+4 format to 5-digit ZIPs, the ZIP12345 will appear
as 00001-2345. Pretty useless, in fact.

2. When Word gets the data from Excel,
a. DDE will retrieve the data /as you see it/ It doesn't matter whether you
see 12345-6789 because you entered 12345-6789 (as in (c) or applied a ZIP+4
format to 123456789 (as in (b)).
b. the default method Word 2002 and later use to get data from Excel (an
"OLE DB provider") gets the data /as it is stored/, but with a twist. i.e.,
if you entered 123456789, Word will retrieve 123456789 whether or not it is
formatted as a ZIP+4. If you entered 12345-6789, Word will retrieve
"12345-6789".

The twist is that when it gets the data, the OLE DB provider tries to assign
a data type to the entire column. In other words, it sees the whole column
as numeric, or as text. Roughly speaking,
c. if you entered all your ZIPs as numbers and applied ZIP and ZIP+4
formatting, the provider will decide that the column is numeric
d. if all your ZIPs are 9-digit ZIPs that you entered as per 1(c), the
provider will decide that the column is text
e. if there is a mix of types 1(a), 1(b) and 1(c), the provider will decide
on the contents of the first 8 cells in the ZIP code column. If they are
/all/ numbers (types 1(a) and 1(b), the provider will decide that the whole
column is numeric. If /any/ of them are texts, the provider will decide that
the whole column is text.

Then the provider has to decide what to do about numeric values in a text
columne, and text values in a numeric column
f. If the provider decides that the column is numeric, and there are no
type 1(c) ZIPs, then we are OK in the sense that all the ZIP data gets
through to Word as 5- or 9-digit numbers
g. If the provider decides that the column is text, we are OK because the
provider then appears to use the display text for all the cells. As long as
they all look like 5 or 9-digit ZIPs, that's what Word will see
i. However, if the provider decides that the column is numeric, any 1(c)
type ZIPs will be lost as they are passed to Word as the numeric value 0.

3. If you got this far, it is perhaps worth asking
a. How could we avoid situation 2(i)?
b. If we manage to avoid situation 2(i), how can we display all the ZIP
codes in Word correctly?

4. I /think/ you can fix 3(b) using the fields I suggested elsewhere, i.e.

{ IF { QUOTE "{ MERGEFIELD zip }99999" \#0 } < 0
"{ MERGEFIELD zip }"
"{ IF { MERGEFIELD ZIP } > 99999
"{ MERGEFIELD ZIP \#"00000'-'0000" }"
"{ MERGEFIELD ZIP \# "00000" }" }" }

There could well be a simpler formulation but at the moment I am only trying
to think of /a/ way to do it.

5. As for 3(a), it might be better to ask an Excel expert (although I am not
convinced that they have to cope with this either unless they want to use
their data in a merge). A lot depends on how you are acquiring your data (is
it in spreadsheets prepared elsewhere, or by you?), whether you prefer
trying to attain consistency during data entry or whether you prefer to
post-process any column that contains ZIPs, and whether you prefer
enforcement (which probably requires a lot of code that you could do
without) or encouragement to conform to standards.

Because of the problem that the OLE DB provider can have with mixed data
types, I think that any solution should aim to end up with an Excel ZIP
column having either /all numeric/ ZIPs or /all text/ ZIPs. Once the data
has been entered and there is potentially a mix of numeric and text data, I
think the only way to achieve this is to use approach 1(d)iii above - i.e.
change the entire column to text. 5-digit numeric ZIPs will become 5-digit
texts, 9-digit numeric ZIPs formatted as ZIP+4 will become 9-digit texts,
and 5-4 text ZIPs will remain as 5-4 texts. You would need the complex Word
field code in (4) to deal with that column correctly.

If you prefer to stick to numeric codes, a good approach might be to try to
encourage people to enter 5 digits or 9 digits and avoid entering 5-4 texts.
There are a couple of ways you could consider doing that, e.g.
a. Select the ZIP column, go to Format|Cells|Number, select Custom, and
enter something like the following (or select it if it is already there):
   [<100000]00000;[>99999]00000-0000;[Red]"wrong" @
b. Format the column header as General or Text

Numeric 5 and 9-digit ZIP codes should appear correctly, and any Text ZIPs
should appear in Red with "wrong " in front. Or...
c. use Excel conditional formatting (which lets you apply colour etc. to
the cell).

Or I suppose you could decide to enter all ZIPs in two columns - 5 digits in
the first, and 4 digits in the second, blank if it's a 5-digit zip. Then
piece the ZIP code back together in Word, e.g.

{ MERGEFIELD zip5 \#00000 }{ MERGEFIELD zip4 \#"'-'0000" }

And so on...