Configuring Safari
Safari uses the global proxy setting. Open System Preferences and then select the Network settings. Click on the Advanced button and then select the Proxies tab.
Change Configure Proxies from manually to using a PAC file.
Enter http://proxy.obs.carnegiescience.edu/proxy.pac into the PAC File URL box.
When you first visit a web site that is directed through the proxy server you will be prompted to enter the proxy server username and password.
You will have to enter this information once every browser session, so you probably want to have it remembered unless you are using a public or someone elses computer.