Adobe – Developer Center : Configuring Third-Party File Compare
This article gives step-by-step instructions to enable the ‘Compare with Remote’ menu item (for Mac & Windows 
. It also provides information on where to get and configure file comparison: WinMerge (Windows) and FileMerge (Mac).
NOTE: FileMerge is available free via Mac OS X Developer Tools XCode package, available from http://developer.apple.com/ (free registration).