Hello & welcome charlie !
CHDK is NOT a firmware update, it's a Add-On that don't change the original camera firmware !
The "firmware update" menu item will be there only if a firmware file is placed on the SD cards root folder, either an original Canon firmware update file (see links beneath) or a CHDK firmware-Add-On file (the filenames of such files are always *.FIR or *.FI2)...
If you want to update your firmware PERMANENTLY (from 1.00e to 1.00f which can run CHDK) you have to do this with a Canon firmware update,
more informations you'll find in the CHDK wikia, read here: FAQ: How_can_I_update_the_original_Canon_firmware ?
See also dpreview: Re: A620 1.00f firmware?
(and perhaps this post in the german forum
Many people have done the firmware update without problems, so i think you'll give it a try - anyway, the update to fw 1.00f is the only way you can run CHDK on your camera...good luck !