Arduino Mega - get mac adress
A MAC address must be unique throughout the environment in which it operates, but apart from that is largely arbitrary.  Typically the problem is solved by selling 3-octet prefixes (called OUI's) to manufacturers, who then uniquely assign the remaining 3 octets to individual device units they manufacturer.

However, if you are unable to discovery a globally unique address assigned to your device by its manufacturer, and know that its traffic will be confined to a local network, you can use a privately assigned (ie, "made up") address with the Locally Administered bit set.   An address of the form

02-xx-xx-xx-xx-xx-xx


or any other with the 2's bit of the first octet set will satisfy this criteria as long as you guarantee that it is unique in your system.

Be especially careful not to set the 1's bit of the first octet - if you do so, traffic will be marked as broadcast and so given wider distribution/attention than needed, wasting network capacity and processor time of connected devices.