I would guess that in this context it's referring to the Machine Access Control address.
This is the unique ID of the network interface. Most NICs are hard coded with a MAC address during manufacture, but some allow manual configuration.
A look through the Squeezebox manual seems to indicate that the MAC should be hard coded, as it says "You can find your MAC address printed on a lable on the bottom of the unit", and there are no instructions for configuring it.
I suggest you contact Logitech support:
http://www.logitech.com/en-gb/