Yes, the usual double-sided option under "Layout" is greyed out. That's the way Canon wrote their driver for the MacOS, and it's the case for some other printer company's printer models as well (hp, for example).
In this case you need to select double-sided in the option a little farther down in that dropdown menu (as described in PAHU's post above) named "Duplex Printing"
For graphical instructions at the Canon website follow
this link, select "Step by step troubleshooting", then choose "How to use the auto duplex printing unit (DPU-10)" then "How to set Automatic Duplex Printing (Mac OS X)"
PB G4, Mac Mini G4, MacBook Pro, MacBook Mac OS X (10.4.7)