It's only going to be available (not greyed out) if your printer can automatically handle two-sided printing. Can it? This is typically only an option on top-of-the-line laser printers.
If it can, make sure you've got the correct printer driver selected in "page setup." You might need to download a new driver from your printer manufacturer. If one is not available, getting much more into the specifics of printer drivers is a little out of my league--GIMP print and all that.
The "poor man's option" is to print all the odd pages, reload the paper in the printer upside down, then print all the even pages.