Play as Michael Jackson

While you are at the main menu, highlight the Arcade option.  Then, press Left, Left, Right, Right, Left, Right, L and R.

Play as Rumble Man

While you are at the main menu, highlight the Championship option.  Then, press Left, Left, Right, Left, Right, Right, Left, Right, Left, L and R.

Play as Shaq

While you are at the main menu, highlight the Survival option.  Then, press Left (4x), Right, Right, Left, Left, Right, L and R.