well you do have to be at the correct power level:
buu - 3million
goku - 1.75 million
gohan - 1 million
krillin - 3 million
piccolo - 1.65 million
frieza - 1.8 million
vegeta - 2 million
cell - 1.9 million
Z button = ascend/X button = descend
its all in your manual in your esf folder