One possible reason is that you're missing a file called "Adobe Unit Types". Open the folder "/Library/ScriptingAdditions/" and look for it. If it's not there, you need to get it back, either by reinstalling Photoshop or getting it from an old system that had it. This happened to me once when I upgraded from Panther to Tiger and didn't transfer all my add-ons.
Some sites have Adobe Unit Types available for download just because this problem is so common. This site has it:
http://www.creativetechs.com/tips/quicktips_31.html . I think it ought to work, assuming there's no version incompatibility.