Bought Dawnguard on steam, loaded up Skyrim and in Whiterun the guy came up and gives me the quest "Dawnguard" with the instructions being "Speak with the leader of the Dawnguard". So I go to where the map waypoint tells me which is just South East of Stendarr's Beacon but there is noone there. I do see Fort Dawnguard to the east but if I try and go there it tells me you cannot go this way.

Any ideas?

EDIT Nevermind found that stupid little crack. Waypoint was way off

LOL yeah, they call it the a** crack, 'cause that's how you feel once you finally find it! =P

Yeah, the maps and waypoint markers for Dawnguard are all over the place for the most part if you're "outside" of Skyrim. Makes places like the Soul Cairn a real pain to explore properly for those of us who have turned off the HUD compass. In my game I added a mapmarker at the entrance to Dayspring Canyon - I was surprised that Bethesda didn't even bother with that.

could someone lease describe where exactly this crack is cos trying to start the dawngaurd quest isdriving me bonkers and i am i to assume the same is for the x box version as i also canna find the leader at the map marker

The Clairvoyance spell really helps on this one because where the map markers point ya is not the access point.
Map markers point way south of it.