Okay so I downloaded Dawnguard this morning and spent my time completing the companions questline for the werewolf and decided to finally start Dawnguard where I followed the quest market to a bit Southeast from Stendarr's Beacon and there is nothing there.

I can't approach Fort Dawnguard ether as when I get near the gate it tells me I can't go that way.

The quest marker also won't appear on my Compass.

How do I get this quest moving?

Okay figured it out. Map marker is an error.

http://gamerant.com/skyrim-dawnguard-dlc-walkthrough-bethesda-robk-156926/

if anyone has the same issue use the video you can warp right to the Riften Stables.