Once you have followed the previous instruction to get access to the CHDK Main menus, i recommend (personal preference) that once you have Alt on the screen, press the menu button, scroll down to CHDK settings, then scroll down to <alt> mode button and change it to 'help'.
In future you will press the question mark(help) to activate Alt menu. I find this less confusing as it just seems more logical to me and as i operate many different powershots i tend to set them as similar as possible.
If you are having trouble in running CHDK in the first place don't forget to start your camera by pressing the playback button, not the power button. (unless you have set up your card to be bootable).
