Unless you like the idea of sharing a Cell with some Guy known and "Big Mary" I'd suggest you take it to your local Police and hand it in ASAP, the PIN code means it's probably being actively tracked.
So you bought what you must have known was a stolen laptop and you are expecting help on this forum. Good luck with that!
If I were you I would delete your post and your user ID on this site and get the hell out of it before the police start investigating. It's never clever to admit to a crime on a public forum.
(Oh, and you'll probably want to ditch that laptop asap. If the legitimate owner has been sensible it can be traced fairly easily.)
Edit: I've just checked a bit further. The fact that the Mac is asking for a PIN indicates that the anti-theft tracking software has been activated. It's probably too late now, so expect a knock on the door anytime soon.
Unless you like the idea of sharing a Cell with some Guy known and "Big Mary" I'd suggest you take it to your local Police and hand it in ASAP, the PIN code means it's probably being actively tracked.