Well, you haven't been very specific, but I'll give it a go.
1. If you go ingame but you can't move, that's probably because Mischief Makers uses the D-pad, so configure your current input plugin accordingly.
2. If it doesn't go ingame and pops up a message about being stuck in a permanent loop, you've probably got a bad rom. Download GoodN64 (and possibly a frontend) and verify Mischief Makers with it. If it's bad, it'll come up with [b1], [b2] etc. at the end of the filename. Likewise, if it's good it'll have [!] at the end of it. If you got the rom in one of those files that has all versions of that rom, find and execute the one that has [!] after it manually via the Open ROM option in Project64.
If all that didn't work, give us some more information about your system, the error etc.