I just started using last.fm and ran into a problem pretty quickly. Not knowing how to integrate last.fm with MC (but knowing it is possible) I started out by downloading the official last.fm software. Since it didn't recognize MC I figured I must need a plugin. I then noticed the extras page on the last.fm site (
http://www.last.fm/download/extras/) and in there the MC plugin, so I installed that. It seemed to install fine so then I went to check out the plugins list in MC and saw there was one there for last.fm (great, it worked!).
Those of you who are familar with last.fm probably already know the punchline. That last.fm plugin I saw in MC was actually there before I installe the one from the last.fm site. What I actually installed was a plugin that is labled Audioscrobbler.
The second punchline is that apparently the old Audioscrobbler plugin is installed such that it is always running (not just when visible) and this has some negative interaction with the last.fm plugin that causes MC to crash. After uninstalling the Audioscrobble plugin things are now fine.
So, I'd recommend that MC ask last.fm to remove the old plugin since it probably isn't useful to anyone anymore.