First go to Porforward.com's Router Index.
Scroll down to find a large list of pretty much every router out there.
Select your router model form the list (If your router model is not there, choose the closest thing to it, and it should be pretty much the same). You will be sent to an interstitial advert first, which you can skip by pressing "Click here to skip this advertisement..." at the top-right of your screen.
Now you will be landed on a page full of pre-defined applications which portforward can help you forward ports for. Find "Wolfenstein Enemy Territory" on that list and click it.
You will now be instructed on how to forward the correct ports for W:ET on your specific router. Instructions are easy to follow and should work perfectly.