1.0.4 - cannot choose the port - only COM (my Uno connected per USB)

Hi,
please can you help me?
I am runny Arduino 1.0.4 under Linux and what can I see is the "Arduino Uno on COM1" in the right bottom corner. But I have connected Uno per USB and my port is ttyACM0. How can I change it?
I have tried to flash the program but it says to me, the port COM1 couldn't be found.
Thanks

In the fourth tab (it's called 'extra' here), you can choose 'serial port'

There is this line with serial port, but this line is grey (not clickable) so I can't choose anything.