Airpods connected to iMac, but not in the audio output device menu
From this siteyou should do the following.


Make sure Bluetooth is on.
with both air pods in the charging case, open the lid and hold down the button on the back of the case.
The green light in the charging case turns white.
Go to the Bluetooth menu item and select air pods and hit connect.


Now air pods should show up in sound Systems Preference panel.