I use kaleidoscope app (http://www.kaleidoscopeapp.com) as me diff app but I couldn't make it work with sourcetree.
Sourcetree has predefined external diff app list. Also I don't have any of those tools so the external diff app section is disabled for me. Basically I can't do external diff right now.
Can you help me to set it up or if it's not possible at this time, is there a plan to add it on the roadmap?