I'm having the same problem. I need to move a post to another blog on the same website.
Update: I talked with squarespace support today via live chat. Apparently the move option is not available if you are trying to move only one post due to a "technical issue" that SS has noted and are currently working on although no estimate of when it will be fixed.
In the meantime, there is a work around as the shift-click move option will appear if you select more than one post. Create a blank post, move both posts and then delete the blank post.