Tried everything you said. I still can't dimension in the design layer at all. The original file I was trying to dimension was imported from some version of AutoCAD, however that doesn't seem to matter, since I can't even dimension a VW file I created myself.
I also did create a new file and send it to a friend, who was able to dimension it just fine. He also sent me a file that he had put dimensions in, but I was not able to add my own to it.
We're thinking there is some kind of bug with the software I received. Is this a reasonable assumption?
Thanks for your help so far.