Dell 3000cn Color Laser Printer User Manual page 49

NOTICE: In steps 4 through 5, the duplex module must be supported until the support straps have been fastened.
4. Insert the hooks on the optional duplex module into the grooves on the back side of the printer.
5. Attach the straps to the left and right side of the printer, and then close the optional duplex module.
6. Tighten the screws on the bottom left and right while firmly holding the optional duplex module in a closed position.