Same issue in Interiors. I fear that this is probably "as designed", aka "crippleware" of the Home Designer series. The Chief Architect professional series (*not* Home Designer Pro) can do this - I found a knowledge base article for that series that shows a cabinet dialog that says "front/sides/back", whereas in H.D. Interiors it just shows "Front" in that dialog. One can rotate the cabinet to *look* at the back in the "Base Cabinet Dialog", but you cannot select it or modify it. In looking at the product comparison chart, this is not an area that lists differences in the various Home Designer line. (Personally, I think this really sucks).
