The MAC address is the unique identifier for your Myo armband. Typically you'll only need to know it if you're directly working with the Bluetooth connection or customer support asks you for it.
You can find the Bluetooth MAC address of a Myo armband two ways:
You can take a look at the label on the bottom of the box Myo was packaged in. The MAC address is shown on the bottom.
Or you can connect your Myo armband to your PC or Mac and head over to diagnostics.myo.com. The MAC address is located at the top of the page.