Seems to be the same on the Receiver. It's in the "About Receiver" menu. I think the field labeled "ID" is the MAC address. It looks like a MAC address anyway.

Of course, if your receiver isn't connecting the server, you can't get the "About Receiver" menu at all. But I could swear it puts up its MAC address if it doesn't connect. Doesn't it? Anyone?

If it doesn't, then temporarily plugging into a hub instead of that mac-blocking router will get you far enough to see the About menu.