Canon PIXMA MP990 duplex print option can not be selected

Last week my old printer, after 6 years of heavy duty, definitely went nirvana. I replaced it with the Canon AIO PIXMA MP 990. After attaching it via USB to my MBP, Snow Leopard (10.6.4) downloaded via Software Update the appropriate drivers. So far everything went fine. The printer and the scanner unit were recognized and I could scan and print. What I discovered however is that the option for duplex print can not be activated. Just to make sure the MP990 is OK I booted via Boot Camp Windows 7 64 Bit and there the duplex print option can be selected and also works fine. However I boot Windows maybe once per year and would really like to know why duplex print is disabled and can not be selected under MAC OS X.