If anybody owns a hard copy or downloaded copy of Bioshock 2 all you have to do is use your game code to activate the game within Steam. Just enter your code within Steam's 'Enter an actiivation code' text box and it will start downloading. I just entered the number from my DVD copy and it's installing now which means I get the DLC for free.
Most of the DLC is actually multiplayer which I'm not interested in but there is one single player item and besides, it's been a few years since I played BS2 so I may give it another run through.