Durak is shooting a crossbow which is the first time possibly that a player could see one. While following Agmaer you will see a character named Durak. He will lead you you to Fort Dawnguard where you can speak with the leader of Dawnguard.
He is more then happy to chit-chat about how he wants to join Dawnguard to kill some Vampires. You will meet up with a nice guy named Agmaer. The waypoint will bring you to Dayspring Canyon and the beginning of your journey. The way point to enter into the new area of Skyrim will be just ahead. Head down the mountain and stick to the right side.
If you have traveled across Skyrim pretty heavily before heading to Dawnguard for the DLC then the fastest point to travel to is actually Stendarr’s Beacon.