You get it from Steam.
Yes, you can.
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 DLC for PC is out as of 8/2/2012 in the Steam Store.
Xbox 360 has been you for a while. PC is out now PS3 is not out that I know of.
Dawnguard will be released on the Xbox 360 on June 26 and for the PC on August 2. Because of performance issues, release for PS3 is still postponed.
It is out already for Xbox 360. The PS3 and the PC version have not had their release dates announced yet.
you can download off of the xbox marketplace. search for skyrim and it is under add-ons
Eventually it will be available on the PS3 and PC. For now it is exclusive on the Xbox 360 for approximately 30 days.
Dawnguard was released for; - Xbox 360 on 26th June 2012, - PC on 02 August 2012, - PS3 on 26/27 February 2013.
PS3 game and add on downloads can only be made from the Playstation Store and not every game is available
No, it is paid-for DLC.
Why wouldn't you be able to just download it from a PC? If you want to know if you can download iTunes for a PC, yes is your answer.