slow_down,
I'm at work now, so don't have access to my home computer with mp3tag loaded, but if I remember correctly, to access the hidden tags first click on the tracks you want to change, go to the "view" drop down on the top menu bar, then click on "hidden tags" and you should see the "ALBUMARTIST" field, among others (at least that's how it worked for me).
To change the field, select the field you want to change, click on the little picture of a pad on the right side, make changes, and then save. I'm going from memory, so you may need to play around a little.
Hope this helps.
Mike Fox