I tried to get mine working a good few times and struggled with all the information. I actually came here to ask for help, and just as I was getting to the end of the post (with the camera in my hand) I got it going.

I thought I would pass on a quick guide for this as a thank you to the guys who've written it for us.

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My setup

The camera has Canon DIGITAL IXUS 100 IS written on the front.

It has a 16GB SD card in it and I am using Ubuntu as my OS.

Use a card reader for this. It might help if you print this out and strike the stages off as you go to make sure they're all done.

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GO!

If you have data on the card you want, put the card in your reader, create a new folder on your desktop and dump the files from the card onto the desktop. Grab a photo or two of ANYTHING at all with the camera and drop that on the desktop, you'll use the photo further down.
Go to Applications / Other and open GParted Partition Editor (you'll have to type in your password). Click the drive tab at the top right of the window. There'll be some things there like /dev/sda, these are the storage devices connected to the computer; phones, cameras, hard discs, pen drives, MP3 players, cards in the reader. Look for the one with a size that matches your camera's SD card. Since mine is a 16GB card, the storage space /dev/sdd (14.94GB) is the one I'm after (it'll probably register as slightly less than the manufacturer's stated capacity. Click it.
On that horizontal 'bar graph' picture, those are the partitions on the storage devices. Right click on any you've got there and click delete. It'll all go blank.
NOTE, if you're card is over 4GB (like mine), you'll need to create two partitions here. If it's 4GB or less, you can do all that as FAT16. I'll assume yours is also a 16GB.
Right click on that blank bar and click 'new'.
A menu will pop up 'Create new partition'. Where it says 'New size' type '16' and where it says 'File system' select FAT16. Click 'Add' and it'll appear on the blank bar.
Click in the remaining (big) grey chunk and click new. The 'New size' will be the remaining space on the card, so leave that as is. This time, select FAT32 as the file system and click add.
Look at the table for the new partitions; underneath the bar. The top (16MB) one is the one CDHK is going on. Look at it's name. /dev/sdd1. Yours may be different, so keep a note of your own. It's easy to forget and start using mine.
Go to Applications / Accessories and open Terminal
Type 'sudo umount /dev/sdd1' (remember to use the /dev name you noted in the last step, not mine!). This should unmount the partition. It won't show you any information, it'll just give you a new command prompt.
Copy and paste this in, minus the bunny ears 'echo -n BOOTDISK | sudo dd bs=1 count=8 seek=64 of=/dev/sdd1'. Change 'sdd1' to whatever your partition was called in GParted!
A few lines of information about copying and reading should appear, instantly, and it'll go back to a new command prompt. That partition should now be bootable by your IXUS 100.
Now click this http://www.zenoshrdlu.com/acid/acid.tar.gz and extract it onto the desktop.
Go to Applications / Accessories and open Terminal. Type 'dir'. You should see a list of folders and files. If you're in your home directory, one of the folders will be 'desktop'. type 'cd desktop', enter.
You should now see the folder with ACID in it on the desktop, probably called 'ACID'. Type 'cd ACID', keeping in mind the terminal is case sensitive.
dir and enter, and you should see 'runacid.sh' as one of the things in there
Type 'sudo bash runacid.sh'. The program should start up.
Browse to that photo you took before formatting the card and click it open. The program 'stalls' a bit for me, so 30s of waiting is a good idea. Eventually, the boxes on the window will fill with the camera's details. Mine registers as a 'Canon IXY DIGITAL 210 IS'. What's important is the 'version' box on the left. Mine states '100c'.
If your card has been in the reader since making it bootable, take it out and put it back in again to mount it.
Click this ---> http://mighty-hoernsche.de/ and scroll down to the IXUS 100 and then pick the corresponding version, B or C, on the left. When the download box pops up, ask archive manager to open it.
Now click extract and dump the entire contents of that archive into the 16MB partition on the card. Check that you haven't accidentally clicked one of the files and only extracted one of them, you want ALL of it on the card AS IS.
Once it's extracted onto the partition, close the windows and open the big storage partition in the file browser.
Right click and create a new folder called 'DCIM'. Then another beside it called 'MISC'.
Close all that, take the card out the reader. FLICK THE LOCK SWITCH ON THE SIDE OF THE CARD!
Put the card into the reader and press the 'play' (review) button the back.
It should now take a second longer to boot and will display a small read message over the cannon logo, telling you which CHDK version is installed.
Try pressing the shutter button to go to photo mode. Does your screen look like someone's been messing with it? Congradulations! CHDK is running!
There are now a vast array of neat options to play with in the menu. To start getting the hang of it, click here ---> http://chdk.wikia.com/wiki/CHDK_for_Dummies#Load_CHDK_onto_the_card scroll down a bit to 'CHDK is loaded!' and have a play with the the histogram.

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Took a bit of fiddling to get it going, but I'm really looking forward to experimenting with this. I just loaded two ten ton skips with rubbish, rubble, broken glass and muck from someone's garden and jet washed the entire thing. I'd have loved to have seen that on time lapse.

Thanks for taking the time to write the kit boys!

CHDK's first test, turning the histogram on and pointing it at a Luxeed keyboard. This photo was taken with a FinePix Z100fd. 😖