I downloaded the Dawnguard DLC but I can't find Dayspring Canyon or any other characters that activate the quests. Is there something you have to do to activate the DLC?
I am playing on the Xbox 360 version.
Apparently, simply hanging around towns and listening to guards will cause the quest to trigger. If you listen to them talk about joining vampire hunters, you will get a quest marker and be able to start the quest.
While it is true Durak of the Dawnguard won't come find you to chat and you won't hear guards talking about Dawnguard until you reach a certain level, you can still Find Fort Dawnguard and talk to the leader of the Dawnguard to start the quest line at any level. Head to Stendarr's Beacon ESE of Riften. Head north from there and you should find a cave entrance along the side of a mountain marked by a flaming brazier. This is the entrance to Dayspring Canyon, home of Fort Dawnguard.
You can more easily find it by leaving the south entrance of Riften and heading east to the back of the city(if the lake is the front) and the road that runs east and west from there. keep heading east past Blackbriar Lodge and you should see the cave entrance on your left(before you get to the edge of the map with the structure on the road separating Skyrim from Morrowind).