Error 'Serial port already in use' Mac

Hi, I'm trying to install my Arduino Uno board but when I try to run a script it says that the serial port is already use but nothing is using it. I've tried moving the rxtx java in the library script but nothing has changed. I've read somewhere that I could use the Terminal but I don't know how to use it.

Any help?

Thanks a lot!!!!

http://www.baldengineer.com/blog/2012/10/08/arduino-fixing-serial-port-in-use/