Warning! How to enable duplex printing is dependent on the version of OSX you are running and the particular model of the printer. This is a general overview, that should be able to get you started.
In OSX there are two methods of enabling duplex printing. Start up an utility/application that provides printing capabilities (e.g. Firefox, Text Edit, Word:MAC, etc.) From the Copies and Pages [Layout] choices' drop down menus, select two sided. Unfortunately, this does not work with all versions of OSX and sometimes duplex printing will need to be configured from Printers and Faxes in System Preferences.
There is no uniformity as to how to enable the duplex option between different versions of OSX. You need to select the printer and the duplex options (in most cases) will be in 'Options and Supplies' or 'Show Info'. To complicate matters even more, where to enable duplex printing, changes depending on the model of the printer, but should not be too hard to figure out from here. For further help you can contact your POC.