After reading this post:
http://community.activestate.com/forum/subversion-external-diff-tool
I've tried using the following command in the "diff options" text box in preferences > source code control > Subversion but it does not work.
--diff-cmd /Developer/Applications/Utilities/FileMerge.app
I figured it out.
1) Download/Install the fmdiff wrapper from here:
http://www.defraine.net/~brunod/fmdiff/
2) In Komodo IDE 6 ( Komodo Edit does not have source code control ):
Preferences > Source Code Control > Subversion
3) Check "uses an external div tool" and set the following Diff Options:
--diff-cmd=/path/to/fmdiff
( Thanks to Tom on the activestate community forum for help with this )