Then you're probably using the obvious button (but it's the wrong one).  You
need to use the Insert Merge Fields button not the big, labelled one to its
right "Insert Word Fields"  It's a common trap ;)

So select the Contacts, Tools, Mailmerge, To email, type subject, OK.
Use the Insert Merge Fields button to access all of your fields including
user defined ones.  On that screen just scrolldown and find the fields and
double click them as required. (click no checkboxes as the preset ones are
perfect)