I just spent a hour going through this so you won't have to. I thought I installed mysql using mac ports but I kept getting this error.
Errno::ENOENT (No such file or directory - /tmp/mysql.sock):Below are the three steps you need to get Mysql running on Leopard for MacPorts.
sudo port install mysql5 +server
sudo launchctl load -w /Library/LaunchDaemons/org.macports.mysql5.plist
sudo ln -s /opt/local/var/run/mysql5/mysqld.sock /tmp/mysql.sock