I don't think it's posts, I think it might be reputation. No one exactly seems to know LOL but it's SOMETHING like that. I only had like 30 or so posts when I could change mine. I seemed to get rep fast. Cause I'm just cool like that I guess.
Anyway just wait for it, it will eventually show up on it's own as an option.
I'll give you some rep, maybe it will help.