... there's something else already using that port. It could be that you already have MySQL started. You could try telnetting to that port to see if there's something there, or you could use the port scan tool in Applications/Utilities/Network Utility to see if that port is open. If so, you have to find out what is using that port (the telnet *may* give you some info) and then stop that process first before trying to start MySQL.
Now, if it actually is MySQL that's already running, then look for mysqld in the process listing (either ps -aex | grep mysqld | grep -v grep) or you can have a look in the Process Viewer appliacation in Applications/Utilities (I think that's what its called, anyway). You should be able to use the mysql utility programms to restart or stop the process. If that doesn't work, then kill it with kill -9 {PID} ... where {PID} is the process ID of the mysqld process.
Hope that helps.
C