Mechjeb doesn't seem to have an interface in version .18. Copied the files over properly, I receive parts, but no interface.
I recently bought KSP, I stumbled across Mechjeb shortly after and decided to give it a try. I've never used KSP .17, I jumped right into .18, and after downloading Mechjeb, it appears it doesn't work with the game, needs to be updated perhaps?
I have the same problem. I thought that maybe i had installed it wrong and even tried reinstalling the entire game. Good to know others are having the same issue.