Once your access point is connected and configured, usually through your web browser, connecting to the unit should be as easy as opening System Prefernces > Network, and making sure you have set the computer to connect via AirPort using DCHP. The good thing about using DCHP is that the system configures itself. The access point will supply the computer automatically with all the settings it needs. To choose the network you wish to join, open Internet Connect in your Utilities folder in Applications. You should see this:

I did the geeky thing and named my home network "madmax." (don't ask). If you make sure the box for showing AirPort status in the menubar is checked you can then select the network you want from the menubar without having to open Internet Connect each time. In that case your menubar will give you this:

. This is neat because if you go somewhere that has a different network, like Starbuck's, you'll see that network in the list from the menubar and you can select it with just a click. When I take my PowerBook to Starbuck's I see the t-mobile network listed and since I have an account with them I can log on with just a click in my menubar. Kinda neat.