Instructions
Open a Web browser on your computer and type your router's IP address into the address bar. The IP address will vary by router manufacturer, so consult your router documentation if you don't already know it. Enter your router user name and password when prompted to log in to the setup utility.
Click on the "WAN Setup" or similarly named option in the "Advanced" section of the main menu.
Select the "Open" option or deselect the "Strict" check box (which option you have will vary depending on your router manufacturer) next to "NAT Filtering."
Click "Apply" to save your settings, then log out of your router setup utility. You can now go back to your game in "Call of Duty: Modern Warfare 2." Since your NAT setting is no longer "Strict," joining matches should be quite a bit faster.