In your bitcoin.conf file you need to set a username and password (and set server=1 so the client starts up as a server not just a client).

on a Mac the bitcoin.conf is in your HardDrive/Users/(username)/Application Support/Bitcoin

If it's not there you'll need to create one. A sample can be found here: https://en.bitcoin.it/wiki/Running_Bitcoin
Just set your own username and password and set server=1