INSTALLATION MANUAL HERE:
http://groups.google.com/group/ml-devel/browse_thread/thread/4f22f19c97839c28
First thing - your camera must be on firmware version 1.1.1 .
You download from the link down in first post, unzip it on freshly formatted card (low level format from camera), delete *.fir files that don't belong to your camera (keep only 500D) and move all the *.pdf somewhere, so you can keep them as a manual. You insert the card into T1i, choose M mode, go to settings, 3rd wrench menu, choose your firmware version, update, after it's done and screen shows text you have ML installed - turn off you camera and turn it on again. Go to movie mode and open ML menu (delete button), go to "Expo" tab, turn Exp. override ON and you can change exposure with Aperture, Shutter and ISO in the very same tab. (You can also press MENU button while in ML menu to show/hide all tabs, even those which don't do anything with current mode).
I, or ML developers don't take any responsibility about your camera.
For more detailed manual/FAQ/info about ML visit http://magiclantern.wikia.com/wiki/Unified .
How did you unzip the download file? It's an infinite loop between the .bin and .cpz
GriffinHelpers 2 months ago