In order to install CHDK on your memory card you need to know what version of the firmware your camera is running internally. This happens to be blissfully easy due to an app called ACID.
ACID stands for Automatic Camera Identifier and Downloader. It was written by Dave Mitchell and is one of three great CHDK-related apps that he hosts. Not only does this app find your camera firmware but it also will download the correct CHDK version for you! Note – you have to move the app onto your computer for this to work. It can’t download anything if you run it from the .dmg.
The next step is to use SDMInst in order to format your card and upload the freshly downloaded CHDK build to it!