I bought this a while ago, but it is back on offer. When I purchased it came as separate Steam Keys for each item as listed below. The keys work for Microsoft Windows, Mac OSX and Linux/SteamOS.
$7.99 = £6.41
The Bioshock Infinite and ALL DLC:
BioShock Infinite: Clash In The Clouds
BioShock Infinite: Columbia's Finest
BioShock Infinite: Burial At Sea - Episode 1
BioShock Infinite: Burial At Sea - Episode 2
Season pass here for £4 for anyone who has the core game already:http://www.macgamestore.com/product/3353/BioShock-Infinite-Season-Pass/