Look at the top-left corner of the Penny screen. That's where the machine number is shown.
Tap that same corner repeatedly, in quick succession, in the exact same spot.
The display will change a few times as you tap. Keep going.
Stop when the MAC address appears. It looks like letters and numbers separated by colons (for example: AA:BB:CC:11:22:33).
Snap a photo of the screen and send it over.
Important: this isn't just three taps. Keep tapping in that same spot until the MAC address shows up. If you stop too early, the screen will reset and you'll need to start over.