If the headset isn’t automatically recognized by the Mac, you need to adjust the sound settings.
Changing the settings in the computer: (2nd pic)
1) Open System Preferences in the Dock
2) From there choose SOUND
To Set the mic go to OUTPUT and choose the headset.
To Set the Speakers go to INPUT and choose the headset.
Changing the settings in FaceTime: (1st pic)
1) Use the VIDEO drop-down menu
2) Under MICROPHONE, choose the headset