That's actually the PRAM and NVRAM reset page with a link to the pre-Intel laptop PMU reset page. I've got no experience with older iMac but I imagine there'll either be a reset button on the logic board or you'll be able to reset it by pulling the power plug out for thirty seconds and pressing the power button and then putting the plug back in. That's just a guess though.
Why does the PMU need to be reset? Try the PRAM reset first to see if that helps too.