Print From Your Computer : Macintosh Printing (Mac OS X 10.5 and 10.6)

Print Double-sided
With the built-in duplexer (some models only), you can print on both sides of the page automatically.
Note:
Make sure you use a paper type that is compatible with the duplexer.
1.
Select Output Settings from the pop-up menu.
2.
Select a binding option for the Two-sided Printing setting.
Note:
If the options for this setting are grayed out, make sure you have selected Plain Paper/Bright White Paper as the Media Type setting. If you’re printing on letter-size paper, select A4 in the Paper Size box, then select the two-sided printing options you want. Then change the Paper Size setting back to Letter.
3.
4.
Click  Adjustments to view more options.
5.