Brother MFC 8840D A - Manual (Page 41)

PRINTER DRIVER SETTINGS 2 - 15
Available Options
In order to use the optional 2nd paper tray, you must install the option
in the driver. Highlight the model number from the Available
Options list and click Add. The optional tray will then be listed in the
Installed box, Paper Source Setting list and the screen illustration
will display the 2nd tray.