MSD 6al issues - bypass ballast resistor
If you read the instruction it says to eliminate the ballast resistor. Doing so will eliminate one potential issues and even if it isn't it will help you in the long run.
Now... I've seen this happen and know from my own frustrating experience that you can experience your symptoms from having the switched power wire to the MSD hooked to a wire that has 12v whent he key is on but not in the start position.
Here's how you check it if you are by yourself...
turn the key to the on position. Go out under the hood and jump the starter soloniod. If it fires, bingo. Find a new wire that has 12V in both the start position and the on position.
If it doesn't, call MSD.