Go to the first level in adventure mode, where you find Diddy. Locate the big pad with King K. Rool's face in front of Funky's Place. Stand on the pad and press Z. You will be transported to a "battle arena", where you must defeat a group of small enemies. Once the timer runs out, you will receive a crown and have ability to play two multi-player games -- a third-person death match-like game, and a multi-player version of the Battle Arena.