Dragon Balls appear to spawn after each main battle in story mode. They can be seen on the dragon radar with characters who have it. They'll be small green dots, fights that aren't part of the regular story are blue and the main quest is a glowing icon. Once you've obtained all seven, the trophy should pop. Go the the capsule house and shift right until you see your dragon ball collection, featuring all seven. Select it and it should ask if you wish to summon the dragon, earning you this trophy...