1. You can download awakenings via the marketplace or by purchasing the ultimate edition of the game. If you get it through the ultimate edition it will be installed via a disc and you will have to insert the normal disc to play it.
2. Awakening is a stand alone piece of DLC which adds some extra story for after the main game. The other DLC's will need to be purchased seperately.
I'd suggest buying the ultimate edition as it can be picked up quite cheap now and is an absolute bargain.