I tried to mark a post CW, but I can't find the check box that used to do that. Has it moved, been removed, or is it just my head that's not on straight?
It's not your head. :)
The Community Wiki option has been removed from general use and is now a mod-only option. If it really needs to be CW, I think the way to do it is to flag it for moderator attention.
Found the source:
Update (just noticed this): You can still mark your answers "Community Wiki", which still means "owned by the community" with the same low rep threshold for others to edit (and no rep, only badges, for the poster).