Instructions
Play through the game as any character until you defeat the level called "Castle of Dreams." This is the second level for Terra and Ventus and the first level for Aqua. This will open up the Wheel of Misfortune for Aqua.
Complete the entire game while playing as Aqua. This will open up the Wheel of Misfortune for Terra and Ventus.
Go to the Mirage Arena. The Wheel of Misfortune level is the second level. Select it from the list of available levels to play it.