A friend of mine found a script that brings up OSX’s FileMerge program instead of the text-based file comparisons you get with mercurial with doing an “hg diff”.

1. download this script and make sure its location is in your PATH
2. add the following to .hg/hgrc:

   [extensions] hgext.extdiff =   [extdiff] cmd.opendiff = fmdiff

   [extensions] hgext.extdiff = [extdiff] cmd.opendiff = fmdiff

3. Now type hg opendiff <filename> (hg op is enough), instead of hg diff <filename>

Source