Ok, here's how u get him, before u fight super Buu in Goku's DU, look around a bit close to the red dot, to find a scene with supreme kai entitled "mountains", then fight Super Buu and Broly, then u go to West City and talk to Bulma (u need to have the SS4 capsule) then u fight Gotenks, and then u get to fight Omega Shenron