This happens when you try to upload a sketch?
Try shutting down (not restarting) and then turning the computer on. Run Arduino and try it again.
Do you have other microcontroller/physical computing software installed (e.g. for Lego Mindstorms or Crickets)? Any extra firewall or virus software?
When you run the macosx_setup.command, do you get any error messages? (On some computers, it seems to not work properly.)