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.
try looking at these wiring schematics for your model.
http://www.mymopar.com/index.php?pid=27one is the interior side, the other is the engine side. I don't know what kind of car you have, I looked one or two and it looks like dark blue and brown wires go from the ign switch to the firewall, through the bulkhead and to the ballast resistor (just labeled as resistor and it's located close to the coil).
first step, Find the bark blue and brown wires that went to the ballast resistor. make sure one has 11-12v with key in run, the other has 11-12v with the key in start position.
if they do, just wire both of the wires to the MSD 12v turn on lead, you will have 12v while starting and while in run...
