LOLWUT? I opened up Max and made a little something and tried to export it to a Halflife SMD file, only the option wasn't there. I made sure the plugins were installed and even installed them again. No luck. Even worse, I can't import SMD files either. What gives? I can't model for HL anymore?
Try rebooting, and if that fails, you'll have to reinstall it if you want to model for HL, I guess.
Purely speculative, of course.