The MAC address is the set of numbers which uniquely identifies your network card in your machine. This unique identifier is required in order to license the product to a machine.  

On Windows machines this can be found by typing:

c:> ipconfig /all

and searching for a number in the form of 11-22-33-44-55-66 (it can contain numbers and letters)

On Linux / Unix / Solaris machines this can be found by typing:

% ifconfig -a

and searching for a number in the form of 11:22:33:44:55:66 (it can contain numbers and letters)

IMPORTANT: If you do NOT provide a valid MAC address, the software will NOT run!"