The MySQL Workbench program make some assumptions as to where files are under Mac OS X - specifically the program assumes that MySQL has been installed from the *.dmg downloaded from MySQL. It's basically a GUI frontend to the mysql client and as such is similiar to the phpMyAdmin web-frontend. MySQL Workbench is a nice and advanced GUI frontend to administer MySQL databases.
You'll have to substitute with /opt/local/.
I'm custom-installing MacPorts in /macports instead of /opt/local