You can also try getting it this way, it will unlock Uub as well:
BEFORE you fight Kid Buu, you need to Reconfigure/Edit you skill tray. YOU MUST HAVE THE FIRST THREE, the rest are what I used (successfully).
1. Kaioken
2. Super Saiyan
3. Spirit Bomb
4. Kamehameha
5. Z Sword
6. Saiyan Spirit
7. Concentration
DO NOT TRANSFORM HIGHER THAN KAIOKEN DURING YOUR BATTLE WITH KID BUU, OR THIS WON'T WORK!!!!! You need to beat Kid Buu with the "Super Spirit Bomb. When Kid Buu's health gauge is in the yellow, do the Spirit Bomb. YOU MUST WIN THE POWER STRUGGLE AS WELL, OR IT WON'T UNLOCK UUB!!! Provide that you did it right, Goku will be in mid-air, and turn into a Super Saiyan and finish off Kid Buu with the "Super" part of the Spirit Bomb attack. After the battle, provided you did everything as listed above, Dragon Universe will continue. There will be a red dot at the World Martial Arts Tournament. BEFORE you go to the battle, Slowly fly over the islands to the East/right of the tournament. If you find the right one, either Goku will land automatically or a ??? will appear. After the scene there you should have unlocked the S.S.4 capsule. Now go battle Uub. Beat him, and you will unlock him.
Death is but a door, Time is just a window,...I'll be Back!!!!!