If I'm remembering correctly from my Canon inkjet the duplex option isn't where I expected it to be (there may have even been a greyed-out option elsewhere); I think it's in the same section of the drop-down menu that you set the overprint if you're doing borderless printing. You might try just going through all the sections of the pop-up box to make sure (Layout, Color Matching, Paper Handling, etc).
Alternately, it's possible that you've somehow gotten a driver that doesn't support auto-duplexing. I've at least heard of situations where the default driver didn't support two-sided while the Gutenprint one did. No point in digging around with drivers until we confirm that it's definitely not available, though.
To take a screenshot, hold down Command-shift-3. That will put an image of the whole screen on your desktop. Better, though, in this case: Hit Command-Shift-4, which will turn the pointer into crosshairs. You can now click and drag to select an area of the screen to take an image of. Do this and attach a picture of your print dialogue to this thread.