I just tried to properly format an sd card and put CHDK on it for the first time since I upgraded to Mountain Lion. I ran into a bunch of trouble. Ill put the steps I used to fix it here so that I can save you the trouble!
At first I used macboot but I ran into an error that I could not fix. If you want to use macboot, then jump to the bottom for the resolution.
Next I used ACID in order to look at a jpg from the camera and determine the firmware version in use. It wasn’t causing the error, but I wanted to make sure that I was using the correct version of CHDK for the Powershot A490 that I was using. ACID is pretty slick, and it worked exactly as described.
SDMinst is an app from the makers of ACID that will format your sd card and then copy over the appropriate CHDK files.
Now generally I don’t read very much documentation for apps that have GUIs. These three apps all have clearly marked buttons and that was enough for me. The one thing that would have saved me an hour was that macboot and SDMinst both have trouble doing what they need to do in 10.8 due to permissions. i.e. they both need to be run as root. I’m sure there are people who would be concerned about giving root to an ‘untrusted’ app, but I wasn’t in this case. Dave Mitchell is the writer of all three apps, and he has also written small terminal scripts in order to open them as root. Here is the script for SDMInst. Here is the script for Macboot.
If this is your first time, then here is a tutorial for finding your firmware version, and here is one for installing CHDK using SDMInst