Find out the specific make and model of the router your internet connection is running through and then locate the router's administrative username and password (it's typically along the lines of "admin","password" but you need to confirm this with your specific router). Once you access the router access information locate your network address and type it in your web browser's address bar. Enter in the administrative username and password and then locate the Universal Plug and Play (UPnP) option and enable it by clicking the button next to it.
That will open up your NAT, but be aware that MW2 is terribly finicky and will sometimes SAY it isn't while it is and will sometimes SAY it is when it isn't. The important thing is that you did what Infinity Ward says will work.