Brother HL-3400CN Series User Manual page 193

USER'S GUIDE
5-36
7. Remove the Connection Cable cover.
Fig. 5-42 Removing the Connection Cable Cover
8. Securely connect the printer and the Tray Unit at the rear
with the metal lock plate and the screw. Undo the screw a
little to release the lock plate and hook the metal tip into the printer,
then retighten the screw.
Fig. 5-43 Securing the Printer and the Lower Tray Unit