Put CHDK on SD card and the option will appear. And don't forget to start camera in Play mode, using Play button.
Of course i did so, and it didn't worked.
For the benefit of anyone finding this thread later, I will state here that I have an S100 and the firmware update method of loading CHDK works perfectly.It's hard to tell from the previous posts what you did wrong, or even what order you followed, but if you had a dual partition card like you seem to suggest, then not only is that not necessary (as zeno suggests), it simply will not work. The CHDK port for the S100 contains no code to support dual partitions (as they are not needed). So when you tried to use your dual partition card, you got the message about "insufficient space" because the S100 was only using the small boot partition - it cannot access the other partition.
For the "Firmware Update" menu item to appear in the Canon menus, only two things are necessary. The first is starting the camera by pressing the Play button (not the On/Off button) and the second is the presence of a ps.fi2
1 file (containing CHDK) in the rood directory of the SD card. If you had the ps.fi2 file on the wrong unneeded partition then the firmware update method would not have worked for you.
1
edit : some older cameras used a ps.fir file