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Guide:

1.) Start up MW2 Multiplayer
2.) Start up a private match (any map and settings)
3.) Spawn into the match
4.) Open up the hack program and choose the level and prestige you want, and click hack.
5.) End game and you will now have the rank and prestige that you chose.
This only works for you, you cannot host xp lobbies. 

hope this helped. took me a while to figure it out