Trouble with "serial port in use" mac osx 10.8.5 Uno

Hi there–

I just got an arduino uno, i am trying it for the first time. everything seemed to go smoothly, but it will not let me upload the first blinky light program. It compiled fine, but it says that the port is in use, this is the message:

serial port ‘dev/tty.usbmodemfa131’ already in use. try quitting other programs that may be using it.

i looked in the forums and google, i have tried:
A) substituting a different usb cable
B) Restarting the computer and not starting any programs except arduino
C) poking around for programs that might be using it-- i had pda net, but i don’t think it was active and i did shut it off, but still no.

i can’t find anything else about this problem… any suggestions