Can't do duplex printing!

I'm in an office with four iMacs running OS X Leopard v10.5.8.

We're all using CS4 Acrobat 9 Pro v9.4.6 (which Check For Updates tells me is up-to-date).

We're printing to a networked Xerox Phaser 6180DN.

And we can't print anything in two-sided duplex from Acrobat Pro.

We can print in duplex from every other application on the iMacs (including other CS4 apps) without problem.

But I need to be able to print non-CS PDF files in duplex.

I've done Google searches and searches within Adobe Help.

I've found the File>Properties>Advanced>Duplex Flip Long Edge

This doesn't make any difference.

Please don't tell me to upgrade to CS5 or Acrobat X Pro ... that's out of my hands.

Please do tell me how I can enable duplex printing from Acrobat 9 Pro!