To unlock the buus you must first complete the game with all dragon
balls in possession at the end of the game and shenron will appear and
you can choose a capsule called babidi's spaceship. With it on the main
menu you can select it. Once in it you must choose a stage 1-4. 1 will
be continuous fighting. 2 will be how long you can stay alive for. 3
will be how many hits you can land before you die. 4 is the best because
you must deflect back and forth a ki blast as many times as you can.
Doing well in these will earn a new kind of currency called killis. The
more you get then the more things you will unlock. You will get Dabura
at around 1000. Fat buu at around 2000. Super buu at arond 2500 and kid
buu at around 3600 and you will unlock moves for them in between. In the
World tournament it is really easy with kid buu and this is an easy way
to earn money.