Roku/Roku Express

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"

(This varies based on roku model, the address is always on the back but the settings look different model to model)