To use your special attack you must have a full bar of energy. Then, press B + L or R to do the attack. Note: This takes away all your energy. The following is a list of all the special energy attacks and who can use them:

Goku and Pan: Kamehameha
Trunks: Final Buster
S.S. Goku: Kamehameha x5
Uub: Kamehameha
S.S. Vegeta: Final Flash
SS4 Goku: Spirit Bomb
Super Baby Vegeta: Revenge Death Ball
Piccolo: Special Beam Cannon