Make sure your computer's AirPort card is on (via the AirPort status icon in the menu bar)
Make sure you are in an area served by UCSC-Guest. See Coverage Map.
Select System Preferences from under the Apple icon at the top left of the Menu bar.
In the System Preferences window, select (click once) the Network icon.
Set the Show option to "AirPort; Set the Location option to "Automatic; Set Configure to "Using DHCP"
Optional: If UCSC-Guest is the only wireless network you connect to, go to the "Airport" tab, and specify "UCSC-Guest" under the "Join a specific network."
Click the "Apply Now" button. Close the Network window.
Launch your web browser. You will see this security pop-up.
Click Continue.
The UCSC-Guest Authentication page be displayed with another security pop-up, click Send.