Ill try dumb it down for you.

Only plug in the two Square plugs between the fuses and the big blue thing then turn the machine on

With a multi meter measure

To test +12 volts set multi meter to DC and on 3J6 pin 6 (Red lead) use pin 11 to 15 of J6 for your ground (black wire)  Between 10 - 14v should be seen

To test +5 volts with DC and on 3J6 pins 7 to 10 still using J6 pins 11 to 15 for your black lead 4.9 - 5.2v should be seen