The Canon-supplied MP530 driver doesn't have duplex printing as a print option. Mac OS 10.5 has Gutenprint drivers built-in for a number of popular printers, including the Canon MP500 which is a near match for the MP530 and includes the two-sided option for printing.
With the MP530 plugged into your Mac (or, presumably, available over the network) go to the Add Printer wizard ( System Preferences > Print & Fax > + ). Pick MP530, name the printer something like "MP530 Gutenprint" and under "Print Using:" choose "Select a driver to use...", select "Canon PIXMA MP500 - Gutenprint v5.1.3" and then Add the printer.
The MP500 Gutenprint driver isn't a perfect substitute for the MP530 one, however, so you should do this in addition to installing the proper MP530 driver from the Canon website and adding the printer using that driver with a name like just "MP530" or "MP530 Canon". In a year or two of using the Gutenprint driver I've only once had a print job that wouldn't render correctly (some missing lines on the printout), and for that one time I had to switch to the Canon-supplied driver to get it to work. All the other times I find it very convenient to have two-sided printing available as an option.
The two sided printing isn't particularly fast, but it's a whole lot more convenient than manually printing the odd pages and then the even pages.
If I could be of further assistance, let me know. If this helps or solves the issue, please rate it and give a testimonial for my response