You have to kill the Summoner in the Arcane Sanctuary. After you do that, you'll see 6 symbols floating around the book that opens the portal to take you to the next area. Those 6 symbols are where Tal Rasha's Tomb is NOT at. You have to find the tomb entrance that has a symbol that wasn't floating around where the Summoner was. That'll be the real entrance.

if you look aty your quest log youll see the real symbol too