You placed it in the plugins folder in
C:\Program Files\Messenger Plus! 3\Plugins (part in
italic depends on your language and your drive), exited MSN Messenger, and it's still not there?
This is a common problem with .NET plugins... At least, I think so. As I had the same problem, and someone else too.
I don't know what I did until it was finally there... I think I kept registering the DLL.
Sorry I'm not of much help.