I did the quest where I had to go to the other faction's leaders to ask them to join together to fight Molag Bal, and left to quest giver to level up some more before I took up the quest to go and now I can't remember where the quest giver is.
Some people don't get the quest even when they are at the right level (45?). The quick fix is to group up with someone that is or can go there and travel to them. A friend had to do this...