Starting with Araxis Merge 2013.4283 (released 14 January 2013) its command line utilities are now placed inside the application bundle at "/Applications/Araxis\ Merge.app/Contents/Utilities/" so perhaps SourceTree can be updated to just look for them there? So folks would just need to install Araxis Merge and then choose it in SourceTree with no copying/linking of files.

More info:
http://www.araxis.com/merge_mac/ReleaseHistory.html
http://www.araxis.com/merge_mac/topic_installingfromweb.html

Copied from this thread - https://answers.atlassian.com/questions/72949/integration-with-araxis-merge