Here's a handy little diagram from ERA's site on how the MSD is wired in to the harness. If you look up at the top of the picture you see connection #40, which normally goes to an old style 12v coil. The white wire would normally go to the positive side of the coil and is coming from the ignition switch. It may be reduced by a ballast resistor in an original set up, but it is now connected directly to the little red wire of the MSD box. The orange wire goes to the negative side of the coil and is your tachometer feed.
This white wire is the same white wire that you see attached to the "I" side of the starter solenoid (but the picture doesn't show that). This makes it very easy to check whether your ignition switch is working and whether you're providing 12v+ "turn on the box voltage" to the MSD box. Just turn your ignition key "ON" and if the accessories light up then you know the "ACC" portion of your ignition switch is operating. Then measure the voltage at the "I" terminal of your solenoid. If you have 12v+, then you know the "IGN" portion of your switch is operating and the box is being turned on, and if you get 12v+ at the "S" terminal of your solenoid, when turning the key to the cranking position, then you know your "ST" portion of the switch is working fine. Of course, if you hear your engine cranking then you don't even have to take that last measurement. Short of a broken wire, that's all there is to it. The next step is to just follow the trouble shooting steps that MSD lays out. There is no magic to this, no voodoo, no tricks. It's not even that hard. You can do this.
