I've used SourceTree in the past to look at diffs with the 'external diff' tool. I was using FileMerge but recently this stopped working. I then followed the instructions to get TextWrangler and KDiff3 installed. Neither of these will launch when I hit the 'External Diff' button. ALthough the configuration will accept me selecting them as my diff tool.
I'm using SourceTree 1.5.6 which the App Store says is the latest version.