You only need to map fields if you are using merge fields that rely on
mapping, such as ADDRESSBLOCK and GREETINGLINE, or if you want to use
mapping and use the standardlist of "Address Field" names with the "\m"
switch in the MERGEFIELD fields.

Not sure about Word 2007 but in Word 2003 you can't assign to the name. You
can work around this by assigning one index to another, e.g. to assign the
Address field "City" to the first column in the data source in VBA, you
might use:

Dim objMDFs As MappedDataFields
Set objMDFs = ActiveDocument.MailMerge.DataSource.MappedDataFields
objMDFs(wdCity).DataFieldIndex = 1
Set objMDFs = Nothing

i.e. if you want to assign the data source fied called "Town" to the mapped
field "City", you will probably need to iterate through the
ActiveDocument.MailMerge.DataSource.DataFields collection looking for the
field whos ename is "Town", and use its index value to set the
MappedDataFields.DataFieldIndex.

Peter Jamieson