In my research, the MAC is arbitrary and needs to be set in your code:
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
(from Examples -> Ethernet -> WebServer);
After that, Ethernet.begin(mac); should start the interface with DHCP.
I also suggest taking a look at Examples -> Ethernet ->DhcpAddressPrinter as this can help with bringing up the interface.