You can find the MAC address for your leak detector on the inside of the device.
Separate the top cover from the bottom cover of the leak detector at the solid blue line. The MAC address is stamped on the blue plastic adjacent to the AAA battery compartment. You do not need to remove the batteries to see the MAC address.
Once you connect your leak detector to the Roost Cloud, you can find its MAC address in your Roost app.
From the home screen in your Roost app, tap on the device tile, then tap again to get to the menu below:
Tap on Technical Information, and you can find the MAC address as well as the firmware version of your device: