I can start the quest, and get the overworld map marker (but no compass marker) for "Speak With the Leader of the Dawnguard," There is absolutely nothing at this marker. I've looked everywhere and am standing right on top of it.

I can see Fort Dawnguard on the world map (but again, no compass marker.) So I try to go there, and reach the end of the world at the gate where the road leading to it says, "You cannot go that way." I've tried to circumnavigate it every way I can imagine, and I always get the same thing: "You cannot go that way."

Version: Xbox 360 1.6 title update w/Hearthfire and Dawnguard installed.

What I have tried: Deleting title update and redownloading. Deleting Dawnguard, redownloading. Deleting Skyrim, all DLC, and the title update. Clearing cache. Reinstalling, redownloading DLC, reupdating. Ensuring that the Add Ons are properly listed (they are.) Loading previous saves (I have hundreds and have tried at least 50.)

Nothing works.

How do I access this content?

Go check your other thread about this. I answered there.