Brother MFC-8640D - Manual (Page 121)

7 - 6 USING YOUR MACHINE WITH A MACINTOSH ®
Specific settings
Set the Media Type, Resolution, Toner Save, and other settings.
Press Copies & Pages to select settings you want to change.
Duplex
Printer settings
Automatic Duplex is only available with the MFC-8640D,
MFC-8840D, MFC-8840DN and DCP-8045D.