Use the Show IP entry in the RetroPie system and it will show you the available network adapters, with their MAC:

...
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DORMANT group default qlen 1000
link/ether dc:a6:mm:aa:cc brd ff:ff:ff:ff:ff:ff

@entozoon please don't. You can edit your first post and make it clear it's a Raspbian issue preferably leaving original text in place for reference) (please don't blank your post as it will mess the thread up). You can't delete the entire topic anyway.