To unlock Broly in your second runthrough of Goku's DU, get to the Majin Saga (the last section), and get to the point when Goku is looking for someone to fuse with. Search the southern island chain for a "???", and you will find an SSJ4 capsule. You will need this a bit later. After the scene where Buu absorbs Gohan, search the desert for a "???", and you will see a scene with Hercule. Then head towards the lower red dot, and search for another "???" just north of it. The go back to the dot for a scene with Kabitoshin. Then defeat Buu and you will unlock Broly.
Broly's DU is pretty short, should take you around 20 minutes.