Just found this:

"Americans will receive the online mode automatically in the games main menu from the get go, but the rest of the world will have to use a code packed in with the first shipment of the game to unlock it for free."

Source: http://www.technobuffalo.com/2012/11/20/hitman-absolutions-contracts-mode-now-free-to-play-online/

So if you're not in America, look for a code somewhere in the package, or email if you got it through Steam. I'm in the US, and am playing the Steam version, and though I've not played any contracts yet, I've seen a lot of popups in th egame menu while playing the sp campaign telling me about new contracts available to play.

Frag_Maniac