Fair warning, I suspect this violates any warranty you might have on the camera! It could also possibly brick your camera, but the impression I get from the CHDK site is that is extremely unusual.
Installing CHDK is very simple, and takes just four steps--you can see a comprehensive installation guide here
, or just follow these directions.
1. Find your camera's firmware version
Plug the SD card into your computer and create a plain text file on it called ver.req. Make sure you've got the ability to see file extensions so that you don't end up with "ver.req.txt or something--on a PC you can turn this ability on in the folder options menu, I'm not sure how it's done on a Mac. Remove the card and place it back in your camera. Turn on the camera in playback mode and press the func.set and disp buttons at the same time (or the func.set and down buttons on some cameras). You'll see something along the lines of "Firmware ver GM1.00E", it's the 1.00E or whatever that you need. Write that down and move on to the next step.
2. Install and run the CardTricks software
This program (again, this is for PC, I'm not sure how to do this on a Mac), both makes the SD card bootable and installs the proper version of CHDK on it. Follow this link
, download, install, and run.
Click "Format as Fat" (remember, this will delete everything on your SD card, so save any pictures first!)
Click "Make Bootable"
Click "Download CHDK" and choose your camera and firmware version, download and save the zip file
Click "CHDK ->Card" and choose the zip file you just downloaded
3. Install the time lapse macro (known as a "Script" in CHDK)
In the CHDK/Scripts/ folder on the SD card, create a plain text document called "timelapse.bas" and put the text located here
into that file, then save it. This is the time lapse macro I use, and it's served me quite well so far. Again, make sure the file name ends in .bas, not .txt.
4. Eject the card from your computer, and install it in your camera
Make sure before you put it in the camera, you switch the lock tab on the card to the locked position! From now on, if the card is locked, CHDK will load on your camera, and if it's unlocked, your standard Canon OS will load.