To configure WPA in OSX merely choose the PhysicsWPA network by using the wireless status icon on the menu bar at the top of the screen, and enter your username and password from the wireless access tool. Do not use the troubleshooting tool in the System Preferences application, as it does not understand WPA.
Remember to verify the certificate, it should read 'www.physics.umn.edu' and is signed by a Certificate Authority trusted by OSX.
See this Apple article: http://docs.info.apple.com/article.html?artnum=304482
