Step 1: Download and activate the console hack. More information about that can be found here:

http://www.modernwarfail2.com/2009/11/15/modern-warfare-2-unleashed-the-dev-console-unlocker-for-pc-v1-1/
Step 2: Start a private match, preferably using the console.
Step 3: Once in the game, enter one of the following commands (without the quotes):

“/set g_gametype gtnw”
“/set g_gametype oneflag”
“/set g_gametype vip”

Step 4: Try to play.

Addition (Thanks to Iragequit, See comments of this article): If it isn’t working, try this: after entering the command to change the gametype, enter the command /map_restart

The first gametype – “gtnw” – is Global Thermonuclear War. To play, simply capture the objective: a nuclear bomb. When captured, the bomb detonates…
The second gametype – “oneflag” – is One Flag CTF. To play, capture the flag in the middle and bring it back to your base.
The third gametype – “vip” – is VIP. To play, defend the VIP. Or, if you’re on the other team, kill the VIP.
These gametypes were only tested on Afghan and Favela. They may not work for all maps.”

So it seems we have finally identified the DLC that Activision/IW wanted to pimp to us. It has been right in the game all along.

Oh IW, first the console, then lean and all the other stuff, sometimes I think you do this on purpose, like you know Kotick is a retard and the only way you can pull one over on him is doing stuff like this.
Either that or you think PC users are dumber than the average eggplant.