If you just need to process all records in the data source where Condition =
1, then you should be able to use the query options to do it. How to do that
depends on the version of Word, but in Word 2002 and later you need to get
to the Mail Merge Recipients dialog (which lists the data records), then
select the drop-down at the top of the COndition column - if there are only
two or three different values in the column you should see them listed, and
you can select "1". Otherwise, you may need to select the Advanced... option
in the dropdown and fill in the "Query Options" form that appears. In WOrd
2000 and earlier, you can get tot the Query Options form via the Mail Merge
Helper and fill it in. Although that should work in theory, sometimes the
query options dialog does not work properly, with some types of data source.

The other option would be to use a { SKIPIF } field at the top of the mail
merge main document. In this case, you could try

{ SKIPIF { MERGEFIELD Condition } = 0 }

or possibly

{ SKIPIF "{ MERGEFIELD Condition }" = "0" }

ALl the {} need to be the special field code braces that you can insert
using ctrl-F9

You can't always use SKIPIF successfully - for example, with a label merge,
you would need multiple SKIPIFs, one for each label, and the trouble then is
that the SKIPIF cancels the merge of the current copy of the document, i.e.
loses all previous labels on the current page.