Go to your MW2 directory, there will be a folder called mods, open that and inside make a new folder (something like isnipe3)
Then after you open AlterIW go to private match (or if your hosting a server) alt+tab to console and type fs_game mods/isnipe3 (or whatever you called your folder) and then start the game (or if game is already started then type: fs_game mods/isnipe3;vid_restart)
hope i helped.