You will first open your print Queue by going to Apple Logo > System Preference > Printers > Select CED-LOB-C650i > Open Print Queue... (in picture).

Then click on the gear icon in the printer queue 

Select Options tab and make sure that Duplex Printing Unit is selected.

This setting will make sure that the option for two sided printing is available when sending the document to the printer.

Note if the duplex option is not available then the printer is unable to print two sided documents automatically.