Open up a terminal and switch to root user.
Suppose : xx:xx:xx:xx:xx:xx = new mac address you wan to assign to you box
Characters allowed in mac address: 1 2 3 4 5 6 7 8 9 0 a b c d e f
enter the following:
# ifconfig eth0 down
# ifconfig eth0 hw ether 1a:2b:3c:4d:5e:6f
# ifconfig etho up
Check the new MAC address by using following command