Using an external merge tool does not work on SourceTree I am running OS X.10, Xcode 7 and SourceTree 2.
When I select a conflicted file and Launch an external merge tool, there is a window that appears very briefly... and then nothing happens.
I tried (see this help page) sudo ln -s /Developer/usr/bin/opendiff /usr/bin/opendiff but it did not work (I got the message ln: /usr/bin/opendiff: File exists).
What should I do?
PS:
Here is a picture of the window that briefly appears.

PPS:
Here are the SourceTree preferences:
Assuming you are on latest version of SourceTree (Version 2.0.5.5 (2.0.5.5))

Try adding the following manually to your .gitconfig (or verify that SourceTree has put these exact lines)

[mergetool "sourcetree"]
    cmd = /Applications/SourceTree.app/Contents/Resources/opendiff-w.sh \"$LOCAL\" \"$REMOTE\" -ancestor \"$BASE\" -merge \"$MERGED\"
    trustExitCode = true


Or you could just enable 
"Allow  sourcetree to modify your global mercurial and git configuration files." in Preferences -> General tab

These lines will be automatically updated