In order to pull off a Super Attack you must charge your Ki Meter either halfway or fully. Your Super Attacks will be different depending on your character. There are two ways to do this, one with QR codes and one without. Either way make sure your difficulty is set to easy and that you are fighting Raditz (Score Attack > Saiyan Saga > Raditz).
Non-QR Code Method:
If you want to use the non-QR code method then do the follow: charge your Ki Meter until you get the box that motions you to jump (located above the Ki Meter), when this option appears immediately jump. If Kinect wants to play ball, you will have to building up an Assault Gauge to win the quick-time event. Whether you succeed or fail at filling the Assault Gauge, you will now be in Ki Blast range from Raditz. From here charge your meter halfway (make sure to avoid the big Ki Blasts the enemy launches at you, if he does) and then launch your first Super Attack (when I say first I mean the one in the middle of the Meter). Once your super attack is over, you will still be at the same range. Continue this process as many times as necessary. It will only count if you suceed in hitting the opponent with the Super Attack, so if it says 'failed' it doesn't count.
QR Code Method:
Use the QR code for Kid Buu (unless you have already unlocked him) and the QR code for a permanently full Ki Meter (listed as Unleash Ki). Now just go into the fight and take the stance for Kid Buu's Super Attack (holding your right arm straight up). Then just repeat that step every chance you get and you should have it in no time (obviously the fight will end so just restart and repeat the process).