To unlock Uub, you need to finish Goku's DU. When in the final battle with Kid Buu, hit him with a successful Spirit Bomb attack to unlock Uub. To do this, when you are in your standard form, hit to go into hyper mode, then hit again when you are next to him to start this attack. To make it successful, you need to win the minigame, in which you have to hit when the bar is closer to the top. If you beat Kid Buu in this minigame, the attack will be successful and you will unlock Uub at the end. Uub's DU story is reallyb short. You will only have three battles, and should take you less than ten minutes.