Sounds like the main power wire might have a bad connection somewhere. The MSD 7530 series ignition will still power up off the keyed 12 volt source with the light showing and able to be computer programmed but will not fire the ignition if there is a power supply problem.

I ran into this problem a few years ago working on my car. I had unhooked everything to replace the battery and the positive wire to the MSD fell back behind the battery in the trunk, I had a few other wires going directly to the battery positive side so I didn't see it right away. It was throwing me for a loop as I could power up the MSD and program with the laptop but it wouldn't fire the ignition.