So, I haven't seen any clear instructions for how people can try out the (unstable) development release yet (instead of the older Alpha 3), so here's my guide. Because it is an unstable development release, it could wreck your camera, so do this at your own risk. Please correct me if I get any of this wrong.
1. Start by updating your camera's firmware to the latest Canon version (1.1.3).
2. Because this isn't a final release, your camera needs a little modification made to it so that it can load Magic Lantern from the SD card. This is called turning on the camera's bootflag. When the bootflag is turned on, and a bootable SD card is inserted, the camera will load and run the file "autoexec.bin" from the SD card. Here, autoexec.bin will be the Magic Lantern software.
To turn on the bootflag, format your SD card in your camera, then copy this file to the root of your card:
http://a1ex.magiclantern.fm/bleeding-edge/5D3/5D3-113-bootflag.fir
Put it back into your camera, then run the firmware update option from the menus. The update will turn on your camera's bootflag.
3. Download the nightly Magic Lantern zip here. Unpack the zip file and copy just the "ML" folder to your SD card. Delete any .fir files that are on the SD card. You've now copied just the fonts and other data that ML needs to operate to your SD card.
4. Now you can copy a Magic Lantern development build (autoexec.bin) to the root of the SD card. Building this file is tricky, but NerveGas has kindly built a copy for us and posted it here. Unpack that zip file and copy the autoexec.bin to the root of your SD card.
5. You should now have an "ML" folder and an "autoexec.bin" file in the root of your SD card. That's the data/fonts and the actual Magic Lantern software. All that is left now is to set the "bootable" flag on the SD card, which tells the camera that it should look for and run the autoexec.bin file for us.
On Windows, you can use the EOSCard utility to do this:
http://pel.hu/eoscard/
Select your SD card, tick the EOS_DEVELOP and BOOTDISK options, install no FIR files, and Save.
On Linux / OSX, you can use the make_bootable.sh script instead. Plug in your SD card, then run the script from the terminal with sudo ./make_bootable.sh . Hopefully you know how to use the terminal.
6. Okay, so now you have the bootflag turned on in your camera (so it looks for bootable SD cards), you've marked your SD card as bootable, and you've copied the development Magic Lantern build "autoexec.bin" and the Magic Lantern data files "ML" to your SD card, you're ready to go! Turn off your camera, put the card in, and turn the camera back on. It should boot and run Magic Lantern.
Note: The nightly builds available on the main site do NOT support the 5D Mark III yet. You need to build autoexec.bin yourself or use the version provided by NerveGas instead (as these instructions already mentioned
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Hi, i did all of this (three, times, just to make sure i was not missing something) but it did not work for me!
I was able to install the Alpha 3 immediately, but when i then tryed to follow this procedure, it just did not work :/
I noticed that, after formatting my SD, putting the bootflag fir on it and running the Firmware Update on the camera, it would quickly show a message on the lcd saying "Cound not find ML files!"... i suppose this might be the reason for which all the other steps i then carefully followed did not work.
I thought of running the Firmware Update by keeping both the bootflag.fir, the ML folder and the autoexec.bin on the sd card which i had made bootable with EOScard, but since on point #3 it said "Delete any .fir files that are on the SD card.", i thought of asking you to please help me rather then doing something stupid

Thank you all for the attention and my compliments for this amazing job you are doing, really fantastic!
EDIT: the moment i switched the MODE selctor to M, everything suddenly worked!!! Setting the mode to M is something that was not clearly stated on this thread (and, in fact, the alpha 3 installed and worked perfectly for me while my mode was on Av) but on the CAMERA PREPARATION in the wiki for installing ML, it said to put the mode to M and so i gave it a try... I hope it might help others
