It depends on which iMac G3 you have. On the caddy loaders- to the right hand side, next to the USB ports- small hole for paper clip, the one with a squggly line next to it, not the one with a triangle.
On the slot loading iMac: Next to USB ports again, and with same symbol, but easier to find as it's the button that is recessed, rather than the hard reset button which stands proud.
Guess you're uploading firmware to upgrade to OS9? If so, make sure insert the CD, shut down the mac, hold the programmers button and hit the power button adn do not release the programmers button until the droll horrid tone has finished mouaning at you!
Rgds
Richard
Forevermac.com, the UK's apple Macintosh refurb store. from Apple Mac classic to iMac, we stock the lot!