That shouldn't be a problem on an iMac PPC. Are you sure you are using an iMac PPC? Go to Apple menu -> About This Mac and see if you have an Intel, G5, G4, G3?
If you have an Intel, go to Hard drive -> Applications -> and click ONCE on System Preferences, and select Get Info from the menu. There you can force 32 bit mode, which will be active upon restart.