In Cataclysm, if you don't know where the dungeon entrance is (read: have discovered it with the whole "Discovered [Dungeon Name] Entrance."), you can't queue for it.
I've been a fan of Totalbiscuit only for a short time, but he has some great information, and his podcast is one of my favorites. Well, in Beta, he made a pretty nifty video about how to get to your dungeon entrances. So I thought I'd share it! Definitely a great resource for personal and guild use. I know I've already posted it on my guild's forums. Anyway, enjoy!