Just wanted to follow up, as I found the problem. It turns out that the ability to install ML (at least on my 5D MKIII) is SD card sensitive. I was originally using an 8GB SanDisk card which didn't work with ML, but when I replaced it with a 32GB HP card, I was able to install and run ML just fine.
I have no idea why this should be! I'm not using the SD card for recording, just to hold the ML program. I formatted the original card both in the camera, on my computer, and the card just wouldn't work with ML (although I could use it to hold photo files etc just fine...).
Just wanted to throw this solution out there in case anybody else runs across this problem; try using another brand of SD card!
I have no idea why this should be! I'm not using the SD card for recording, just to hold the ML program. I formatted the original card both in the camera, on my computer, and the card just wouldn't work with ML (although I could use it to hold photo files etc just fine...).
Just wanted to throw this solution out there in case anybody else runs across this problem; try using another brand of SD card!