At first you should confirm that the AirPort is enabled. This might enable automatic, or is also accessible from the menu. If not you'll be able to launch the 'internet Connect application' and click on to turn AirPort On.
Once the AirPort is enabled, you should to go to the Network part of System Preferences. Choose an appropriate location; choose the 'AirPort device' in the show, then the AirPort pane.
To avoid it selecting random networks you will be close to, make sure that By default, join is set to preferred networks, instead of Automatic.
Click on the 'Options', and ensure that the settings of Ask before joining an open network and Automatically add a new network to the preferred network list isn't checked. After that please click the OK button.
Back in the Networks window, click the + (plus) icon, and select Unencrypted.maths.cam.ac.uk (if you are in range it should offer than from a pull-down list). Click OK.
Back in the Networks window you should now see the network in the preferred list. Then you need to set up the TCP/IP, and Proxies panes as for a wired network.
This setup has been tested on Mac OSX 10.4. Configuration should be very similar for 10.3 though some images may be different. For other versions of Mac OSX please refer to the documentation provided with the system or contact your local computer support staff.