I did the following for my iPhone a couple of weeks ago. I believe it's exactly the same for the iPod touch:

* Go into Settings and select the WiFi option.

* If you have the 2Wire router set up to broadcast the wireless SSID (your network name), you should be able to click on that network name and then it should give you a dialog box to enter the key.

* If you don't have the 2Wire set up to broadcast the SSID, you need to click on "Other" under the network names, and it will give you a dialog box where you enter the network name and the key.

Hope that helps.