The short version of it is to type ipconfig into your computer's command line, find out what your routers ip address is, and type it into any internet browser. You'll need to login to the website (the username and password are usually both 'admin'). This is how you get to the router settings.
The exact steps from this point on vary based on your router and isp, but regardless of that you'll need to find out which ip address is your Wii's, and then type the Wii's address into the dmz settings page.
The exact steps from this point on vary based on your router and isp, but regardless of that you'll need to find out which ip address is your Wii's, and then type the Wii's address into the dmz settings page.