There are just two important details when it comes to integrating MyBB:
one: Your site can't have any members besides you, else the process gets really messy (
how to deal with that).
two: The newest version of MyBB won't work. You'll either need to download MyBB v1.6 or tweak a few internal files so you stop getting the 'Failed to create forum account' error while trying to register someone (
see here for details).
This is just one of those questions that keeps getting asked over and over so I wish a sticky would be made for it. *sigh*