You can try

Suppose your test field is called myfield and you want to compare it with
"abc", and include the document "c:\myfolder\mydoc.doc" if it matches. Then
try

{ IF "{ MERGEFIELD myfield }" = "abc" "{ INCLUDETEXT
"c:\\myfolder\\mydoc.doc" }" "" }

All the {} need to be the special field code braces that you can insert
using ctrl-F9. Everything else is plain text. Not the doubled-up backslashes
(although you can probably use single forward slashes instead).

However, you may find that your included file does not actually update
correctly if you do that, and in many situations it's better if you can
"invert" the nesting so you have something more like

{ INCLUDETEXT "c:\\myfolder\\{ IF "{ MERGEFIELD myfield }" = "abc"
"mydoc.doc" "empty.doc" }" }

where empty.doc is an empty Word doc. However, the problem with /that/ is
that will get an extra paragraph mark - if you don't want it, what you can
do instead is
a. select the complete text of mydoc.doc and use insert|bookmark to insert
a bookmark called "all" (say)
b. select a single point in mydoc.doc and insert a bookmark called "none"
c. use

{ INCLUDETEXT "c:\\myfolder\\mydoc.doc" { IF "{ MERGEFIELD myfield }" =
"abc" "all" "none" } }