In Moodle 2.0, the glossary pop-up window has been entirely re-designed and no longer features a link to attachments. However, atttachements can still be shown, when browsing the glossary itself.
What kind of attachement did you want to attach: image, audio, etc.? Could you not, instead of attaching, insert it in the Definition field?
Too bad attachments don't work. I think I prefer 1.9 on this point. Too bad. Seems like a step backward.