Once Steam finishes downloading it, it should automatically be activated, you can check this though by looking at the Data Files option on the launcher and ensuring Dawnguard.esp is checked.
Once you are level 10 or higher, the keep guards will start talking about the Dawnguard, adding the quest to your journal.
Once it's all downloaded and set up, make sure the plugin is activated by selecting Data Files when you run Skyrim. Assuming that's all correct, when you load a save, you'll have two additional missions added to your quest log, Sealing the Rift, and Dawnguard, added by the DLC.
Once you have the Dawnguard add-on, if you go to Winterhold, you can find a guy who will ask you if you are interested in joining the dawnguard vampire hunting guild. Do so, and you are started. I'm yet to join it so I can't tell you what it's like.
No, it is paid-for DLC.
It is daedric armour!
It costs £12.99
Dawnguard is basically an expansion pack for skyrim. like all expansion packs, it requires the base game to play. So on a PC (and any other gaming device) you need skyrim first and then you can install dawnguard.
Skyrim for Pimps - 2011 Dawnguard Begins 3-1 was released on: USA: 1 August 2012
Skyrim DLC for PC is out as of 8/2/2012 in the Steam Store.
February 26th 2013.
By purchasing it and installing. Different consoles have different methods.
Dawnguard is the expansion that adds the werewolf perk tree, without that, you can't get werewolf perks.
Yes, its downloadable content for the full game.
The disk version is already out.
There are 13 perks in total.