The MAC should be printed on a little sticker at the back of the box.
Otherwise just connect via telnet and type in "ifconfig -a"
There should be something like this:
ZitatAlles anzeigenwifi0 Link encap:UNSPEC HWaddr xx-xx-xx-xx-xx-xx-xx-xx-xx-xx
BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:195
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Interrupt:33
The xx-xx-xx-part should be your MAC address.
Last chance, open the box, look at the wifi-adapter 
Just switch of mac- protection on your router, let the box connect and then read the max from the webinterface of the router. It should list ot somewhere in the list of connected devices....
wifi0 Link encap:UNSPEC HWaddr xx-xx-xx-xx-xx-xx-xx-xx-xx-xx
Even though the first six bytes of the "HWaddr" of wifi0 are equal to the wireless MAC address by default, the better way to get the MAC address is by looking at ath0 instead of wifi0.
ip addr show dev ath0or
ifconfig ath0To get the addresses of the wired interface, use
ip addr show dev eth0or
ifconfig eth0To get a list of all interfaces and their addresses, use
ip addror
ifconfig -aThe ethernet address is labeled "link/ether" or "HWaddr".
Thanks for clarification!