Mac OS X has had a long and hard history with the UChicago VPN—The last time it worked properly was under Mac OS 10.4, and the Java applet at http://cvpn.uchicago.edu has been dicey at best ever since. There is a download available at answers.uchicago.edu for users of 10.4-10.7, but the version for 10.8 is not there yet. You can download the updated VPN client for 10.8 here but in order for it to work, you first have to disable the "Gatekeeper" function of the new OS.
Enjoy, and happy VPN-ing.
Update: As of 8/21/2012, the new Cisco software has been installed at cvpn.uchicago.edu and should play nice with all current Mac OS versions.