SourceTree does not yet officially support Kaleidoscope 2, so here is how to set it up:
In Preferences, under Diff:
You can try selecting Kaleidoscope, but we have had reports of mixed results. Some people have rebuilt their launch services database and restarted and been able to use Kaleidoscope this way:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain use
