simple...
1. open your *.def file.
2. select the tab "Group"
3. create a new tag with the id "entity"
4. inside that tag group create a new tag called "item" and one called "unit"
5. you're done
basically what (i belive) is going on is this....
ABCreator allows you in teh data file to make units / items / options.... however, each unit / item is designated as such with an "invisibale" tag that you do not see. This "invisible" tag is referanced at run-time so it knows how to handle the various "widgets" as either units or items
or....
you need this tag group in your def file for armybuilder to work, even if you never use it for anything.
i hope this helps you out.