Howell,

Every think I have learn, about procedure has been through trial and error.  

If you need to start APU, you must turn on aircraft Master Battery Switch on.

I suggest open your  FSX Free Flight Screen "Load " and go to "Cold -n- Dark start at KSEA.
Now your airplane at KSEA airport  and all powers shot down.

Before touch any thing go to APU and click APU start switch. APU will not start. Because you need to open  Aircraft Battery Switch on.

Now plug in External Power you will see some light will be on in FE panel. Go to APU turn on APU starter Switch. Nothing will happen.Because "Battery Switch" still OFF. Turn switch on... :D
Have a nice fly... ;D ;D