Ubuntu; Arduino serial port not found

I'm on Xububtu 13.10 64 bit. I installed Arduino IDE from the software. I have Arduino uno board. when I try to execute any code, I get this error

Is there a driver missing?

Well, did you choose the right serial port ?

I'm using an Uno with Ubuntu 12.04

The Arduino IDE has a Tools menu which lets you cselect the serial port.
I find that if I've unplugged the USB to the Arduino, next time I try to open the Serial Monitor it tells me I've selected the wrong port. Then I select another port and it works.