in Goku's DU, second time through, before you fight super Buu as Vegeto, look for a scene near the red dot entitled mountains, after you've been there, go fight Buu, after that, you'll get a new red dot wich will be Broly. after you defeat him, go to the chain islands to get the SS4 capsule, then talk to Bulma in west city before you fight Gotenks, and when you've defeated him, you'll fight Omega Shenron.