I see this box on some articles I post to Discussion, but not others.
Moot point now, but for future reference: the last time this happened to me, I tried going to the edit URL as though it were already on the main site (i.e. http://lesswrong.com/edit/3gi instead of http://lesswrong.com/r/discussion/edit/3gi), and that made the “post to” menu show up.
I see this box on some articles I post to Discussion, but not others.
Moot point now, but for future reference: the last time this happened to me, I tried going to the edit URL as though it were already on the main site (i.e. http://lesswrong.com/edit/3gi instead of http://lesswrong.com/r/discussion/edit/3gi), and that made the “post to” menu show up.