before the finnal battla fighting vegeta mr. Popo will ask you to got all the dragonballs. Then one of the wishes you will have a changes to fight Broly. But be warned he is very ,very powerful! He is in East Mt. Paozu well good luck I tryed betting him and its hard.
Before the last battle against Vegeta you will be asked to vist Mr.Popo who tells you that he can make his own set of Dragon Balls, by doing so you'll receive the chance of choosing from several wishes and unlocking Broly is one of them, but be warned be is very very powerful even a team of level 60's can't stand there own so train through out the surroundings up high in East Mt. Paozu for it is the strong area in the game.