Board Thread:Mod Discussion/@comment-26818513-20150815091549/@comment-6881629-20150815092911

You should have the quest Dawnguard go do that to start the dlc's main story and before you actually join the Dawnguard fully you'll get the offer to join the Volkihar instead.

If you don't have the quest when your about level ten guards should start talking about it and Durak the recruiter should show up, but you can just go to Dayspring Canyon and Fort Dawnguard without it.