You can control manual double-side printing (i.e., you physically turn the sheets over) using the Paper Handling setting in the print dialog. In case you can't select that option, here's how you can manually print double-sided pages.
If not, or if that option is unavailable, you can't. If that is checked, you can use the automatic double-side printing (by checking the Two-sided box next to Copies). In there, you should see an option for Duplex Unit (if your printer supports it). You can go to Print & Scan in the system preferences and select Options & Supplies for your printer.
