You need to install the Duplex unit (DX-7000) on your machine.
This is an optional device that allows you to print on both sides of a piece of paper. When it is installed, you can select either 2-sided(duplex) printing or simplex printing on the control panel or with a software command.
To see how to make a setting to 2-sided(duplex) print on the driver or control panel, click here.
To install the duplex unit, please see the User's Guide that we have supplied with the Duplex unit.