WordPress version: 3.4
WordPress SEO version: 1.2.3
At "Titles & Metas > Taxonomies" I activited the Meta Box for Tags. But the Meta Box does not appear on the Tag's edit page. I haven't tried shutting down other plugins or changing the theme 'cause I don't think that would cause this 'problem'.
I'll add some code to the theme so it uses the Tag's description as the Meta Description. Not a permanent solution, but better than no Meta Description.