It depends on where you bought it from, honestly. If you bought through a vendor that is not us, you just go to that vendor and typically it's right there in your account.
If you bought from us, you just send a note to arcengames at gmail dot com, noting what email address you bought the game with, or your name, and we'll send you the keys.
Actually we get like 5 of these a week, you'd be surprised.