First you'll need to download the WebTV Viewer. You can get it here.

Once you have the WebTV Viewer, open it and you'll be brought to Connection Setup. Make sure you select the "Custom" option, then enter the following details (make sure to note them down, you'll need them every time you connect to WebTV Redialed!): Address: 23.247.14.250 Port: 1615.

When you're done, your settings should look like this:

Once your settings are correct, click "Connect". If you get a message saying "There is no Web page or other information available for the item chosen", type "client:active" into the URL box and hit enter. With any luck you should be taken to registration.