How do you unlock king mickey on kingdom hearts 358 days

After you beat the game, check your holo-missions and make sure you completed every mission, there will be the word done next to each mission if you have. After you've complete every mission, go to the Moggle Shop outside the Clock Tower, he will tell you that there is a new item called Return Of The King. Buy it, and you can use Mickey Mouse in mission mode!
Good Luck!!!