The MAC address is accessed by swiping left to Settings, then About and the MAC address is there. I'm not certain exactly where you are in the Setup process or if it is available at that point. It may be that it is not available until after Setup.
I'm not sure why one would need a Bluetooth Mac Address to setup the tracker.
Yes I believe the MAC address is not available until the unit is setup.
@Uq if you really need the MAC address of the Bluetooth, setting up wifi is not part of the initial setup process and may be skipped, you will need to contact Fitbit with your serial number found on the box.