I've posted before how to configure git so 'mergetool' uses Araxis Merge, this was however slanted heavily towards a Windows installation. Doing the same for OS X isn't that much different but does require an extra step.

• Download & Mount Araxis Merge for Mac OS X

• Drag Araxis across to your ~/Applications folder as normal

• Copy the contents of the Utilities folder to (eg) /usr/local/araxis/bin (info here)

• Add the path to your startup script: export PATH="$PATH:/usr/local/araxis/bin"

NB: Make sure the command-line files have execute permissions (e.g. chmod -x /usr/local/araxis/bin/*).

Now update your .gitconfig to use the merge command-line file:

[merge]
  tool = araxis
[mergetool "araxis"]
  cmd = araxisgitmerge "$REMOTE" "$BASE" "$PWD/LOCAL" "$PWD/MERGED"

BISH BASH BOSH!