A MAC is 16 characters that uniquely identifies the In-Home Display (IHD) for your smart meter. Different manufacturers use different terms such as GUID or EUI. It is not the serial number.
Where do I find my MAC?
This will depend on the manufacturer.
A - Within the In-Home Display (IHD) The IHD is a small electronic screen that shows the information from your smart meter. You may be able to find the MAC in the settings (or similarly named) section of the IHD. In the example below it is accessed via the Meter Info item and then by scrolling through a couple of pages.
B - On the back or underside, The MAC may be printed on the back or underside of the IHD. If there is a barcode there is an option to scan this in the app as the digits can often be hard to read.
C - In the battery case In this Geo device remove the battery case to find the MAC. The MAC and its barcode are the second of the 2 identifiers.