How can I plug FileMerge into my Source Control App
The issue is that you are setting an app bundle in the config when the version control program is expecing a Unix command line.

You should use /usr/bin/opendiff as the command line. I think that is the only chnage you need for the perforce diff you show.

Example git configs for merging are in this  question and several other duplicates on StackOverflow.