You can go to
Print & Scan in the system preferences and select
Options & Supplies for your printer. In there, you should see an option for
Duplex Unit (if your printer supports it). If that is checked, you can use the automatic double-side printing (by checking the
Two-sided box next to
Copies). If not, or if that option is unavailable, you can't.
In case you can't select that option, here's how you can manually print double-sided pages.
Manual double-side printing
You can control manual double-side printing (i.e., you physically turn the sheets over) using the
Paper Handling setting in the print dialog.
If you select
Paper Handling, you'll find a
Pages to Print option that lets you print either only the odd pages or only the even pages (default is both). To print double side manually, select
Odd Only first and when it finishes printing, flip the pages over and select
Instead of flipping each page over to its blank side before printing the even pages, you have a more convenient option of flipping the entire block of pages, so that the last sheet is on top (useful when you have a lot of pages to be printed). With this, you can then choose
Page Order to print the even numbered pages backwards. (Note: be careful that if you have an odd-number of pages in total, you might have to leave the last one out before printing the even ones.)