The built in integration between the Mac screen sharing application, Safari, and CrossBrowserTesting.com makes using the desktop vnc option seamless when accessing it via a mac. To enable it, click on the check box that says 'Use Desktop VNC Client' by clicking the 'Live Test Settings' link at the top of the live testing console.

NOTE: There is a known issue with the Screen Sharing application on OSX 10.6+ where you may get a white / blank screen. If you see this, follow the simple fix at http://crossbrowsertesting.com/faq/i-am-getting-blank-screen-white-or-black-when-using-screen-sharing-desktop-vnc-client-mac-osx. This fix will correct it permanently, and takes about 60 seconds to implement.