On Mountain Lion, I found that the my.cnf file is missing. This is because Mac uses the default MySQL settings and therefore doesn’t require a config file.
If you want to change the default settings, you’ll need to create a my.cnf in /etc/:
You’ll need to sudo into Terminal, and copy one of the default configs from the MySQL folder to the /etc/ folder:
sudo cp /usr/local/mysql/support.files/my-huge.cnf /etc/my.cnf
Then just use Nano or whatever to update the file, then restart MySQL and you should be good to go with your new settings.