To use Changes instead of FileMerge in Versions for doing file diffs create a symlink to chdiff (the command line tool for running Changes) from the original opendiff path.

First, Become super user. sudo -s cd /usr/bin If necessary, backup the old opendiff binary. Then, create the symlink. ln -s /Applications/Changes.app/Contents/Resources/chdiff opendiff

Make sure you type exit after you are finished.