So how did you figure this out? As far as I know (from reading the datasheet I haven't actually gotten around to playing with my ethernet shield yet), the MAC address for the ethernet shield(s) are something that you have to provide from the arduino sketch, not something that is present on the board (as it would be on a PC NIC card.) All you really need to make sure of is that you don't use the same ethernet address as some other system on your local ethernet, and you don't accidentally use a multicast address (odd first byte.) It probably makes sense to use a "locally-administered" MAC address like "02 02 02 02 02 nn" (where nn is a number you pick. A MAC (Media Access Control) address is a unique identifier assigned to most network adapters.
I hooked it up to an ethernet cable of a laptop I don't use anymore, and figured out the IP and MAC adress. I have a mac address as a string in the following format: xx:xx:xx:xx:xx:xx (six groups of two hexadecimal digits, separated by colons).