Your question would be more proper for the MS Word newsgroup.

However, in MailMerge, YOU select what fields to display in the letter. You
have 100% control over which fields and where they show up. I suggest you
look at the MailMerge HELP in Word for more detailed information.

This is one of the many problems and evils of using what we call lookup
field. this issue's explained here a bit:

http://www.mvps.org/access/lookupfields.htm

This solution to the problem is rather simple, simply fire up the query
builder, drop in the original main table, and drop in and join in the
additional tables for the additional look up values that you need. You can
then choose and drop in the description field and not use the ID fields as
you are now.

so the solution is to use a query in place of the table directly, and in the
query at in those description feels that you need.

The above is a perfect example as to why I avoid and do not use those look
up fields. As you can see at the end of the day to your winding up to use
SQL and writing this in the correct way that you should have done in the
first place...