In the print dialog box, be certain you have selected the PPD (such as LaserWriter 8500 in this instance) in the ppd pull down menu. Once selected you will be able to access these options via the button marked Features.
Clicking on the Features button you will see the option to duplex, but you will need to tell Pagemaker that the duplexer is installed first. Scroll down to the bottom of the list and change duplexer: not installed to installed. Do the same for any optional feeders you have attached. You should now be able to scroll to the top of the features list and choose the various methods of duplexing from within Pagemaker.
If this still does not produce the desired result, hold down the option key when selecting print to access the traditional page setup and print dialog boxes from within Pagemaker. A button marked "options" will appear within the print dialog box to allow you to access the traditional print features.
For additional information see:
TIL Article 24333: "
LaserWriter 8500: PageMaker 6.5 Duplexing Issue"