To locate the MAC Address of your Roku:
On the back on your Roku there are a pair of six 2-digit pairs separated by colons, e.g. 00:0D:0A:1H:3G:5C. One set is for the Ethernet connector and for the other for the wireless connector.
Alternatively, from the Roku home screen, select Settings > Network > locate the value next to "Wireless MAC Address"
On some Roku SmartTV's the MAC address can be found under "Private Network".