Just follow the instructions from Audionut, it's very simple:
INSTALLATION OF MAGIC LANTERN
1. Make sure your Canon Mark iii has the latest firmware 1.2.3 installed.
2. Take out CF card from camera.
3. Format SD card and download Magic Lantern files
builds.magiclantern.fm/#/4. Select platform from drop down '5D3.123'.
5. Unzip the file 'magiclantern-Nightly and copy the unzipped folder contents 'autoexec.bin', 'ML' and 'ML5D3123.FIR' to the SD card.
6. Run the Firmware Update from Canon's menu.
7. After restart insert CF card and press trash can symbol to open Magic Lantern interface allowing a moment for installation.
8. Go to the Modules section in the ML interface and check these fields 'file_man', mlv_play', mlv_rec', mlv_snd' and then restart your camera.
9. The camera will now shoot Raw footage using the mlv codec.