First this section of the forum is for ESF only.
ECX related stuff goes into the pluggins section. (dont people read the stickys)
Secondly. Fusion is a character special. Meaning its used just like any attack after you reach a certain PL. But instead of shooting a beam youll get a new character.
So get enough PL, select fusion from theattacks (number 9), LAND!!!!!, press and hold the right mouse button and get all the arrows that appear correct. If you miss one arrow of the 6 you will fail the fusion and get either fat or slim gegeta/gotenks.
You have about 2 seconds to remember all the arrows before they turn invisible. The way of imputing the arrows is the same as advanced melee.