If you're bnet has the... I think its "character level 50" achievement or whatever, then you have access to chromie time, iirc. Which means your character can simply swap to a different chromie time? Honestly, Im not terribly sure though. Chromie is a stupid, pointless mechanic and I hate it so I dont pay attention to how it works. Maybe someone else can give you a better answer. But you dont have to do anything special to unlock the dungeons. You just cant use the dungeon que tool for them. Just walk into them.