Dell 3100cn Color Laser Printer User Manual page 61

6. Remove the screws of the straps on the left and right of the module from the optional duplex module.
7. Remove the optional duplex module from the printer.
8. Gently push both sides of the bottom of the paper cover to install it.
9. Open the inner rear cover. Attach the cover, which you have kept since installing the duplex module, until it clicks, and then close the rear cover.