Open a terminal window and enter the following line:
/sbin/ifconfig | grep HWaddr
The MAC adress is the numbers and letters to the right of the word HWaddr in the output.
If your machine has more than one ethernet interface, e.g. a WLAN interface, be sure to give the MAC address of the wire interface. Typically this is called eth0 or something similar.
The address is always in the form aa:bb:cc:dd:ee:ff, looking something like “ce:9e:8d:02:1d:e9” or a variation of.