Get enough points with Goku, Vegeta, Goten or Trunks. When you have enough points, choose Fusion dance icon among weapon icons. Stand on ground, press and hold right mouse button. Arrows will appear in left upper corner. You will see them for one second and you'll have to memorize them and press them in correct order for short time (needs practice)
1. If you succeed, you'll transform into Gogeta/Gotenks.
2. If you don't succeed you'll transform into either slim or fat Gogeta. (I suggest you to type "kill" in console in that case and try again).