Instructions
Close "Call of Duty: Modern Warfare 2" completely.
Open the "Players" folder in your "Call of Duty: Modern Warfare 2" directory.
Open "config.cfg" in this folder with Notepad.
Add "self setClientDvar("scr_nukeTimer", "X" );" to the end of the file. Replace "X" with the number of seconds you want the nuke timer to be.
Save the new config.cfg file.
Start "Call of Duty: Modern Warfare 2." The new nuke timer should be in effect as long as you do not play online multiplayer.