Xerox doesn't have a manual duplex option in the driver(s) so you are at the will of the app having it.
Word does, Acrobat doesn't, I don't know of any web browsers that have it.
Basically, your only real options are
1. Print the even numbered pages, re-load the paper upside down and print the odd numbered (A complete pain)
2. Return the 6515N and get a 6515DN (Costly)
Disapointed to learn that my new 6515N does not do automatic duplex :0( - so I am wasting a lot of paper as manual duplex seems hit and miss ? It works from WORD by selecting Manual Duplex - but I don't get this option with Powerpoint, hotmail or when printing from the web? Printer is on a small home netwrk with Windows and OS Mac devices accessing it - Help me save the planet please :0)