Gaieus-

Nope, that's not it. The path to Extensions is different because I'm on a Mac (It's under Google Sketchup -> Preferences), but Ruby Script Examples and Utility Tools were checked when I first noticed the problem. I've toggled the Ruby Script box off and back on to no beneficial effect and have shut down and restarted the program with the same negative result.

Oddly, the Ruby scripts seem to continue to work. For instance, under Draw, I still can choose Box and Shapes, scripts that I had earlier installed. My imagination has run dry about what else to try.