Direct2Drive have ceased offering the PC version of Manhunt 2 in North America since being acquired by the videogame rental service Gamefly. As it now stands, there is no official way of purchasing Manhunt 2 for the PC.
GroveSrTLife received the following response from Rockstar on the matter:
There has been a change in ownership over at Direct2Drive. Gamefly, who is the new owner, does not approve of Adult Rated games, and refused to offer it on their Website now. And we also realize there is now no way for you to acquire this game for your desired platform (PC).
We very much appreciate you bringing this too our attention, so much so that we would like to genuinely show you. Please reply with your home Mailing address for some free swag! A physical copy of Manhunt 2 might even accompany it.
Best Regards, Sam H.
You can see some images of said swag by reading GroveSrTLife's thread
on the forums.