Hi David.

Turn the power ON. Then turn the APU switch to START, it will immediately return to ON. You should see the RUN light come on, briefly, twice before it comes on and remains on. This provides enough power to supply all the instruments.
Alternately. You could press SHIFT+2 to bring up the SimIcons Panel, click on the "E" witch will supply External Power, then press the EXT PWR button (AVAIL should be lit up).

Have you done the tutorial from here (http://www.captainsim.org/yabb2/YaBB.pl?num=1220997876) by aussie? It should get you going. I couldn't get the 757 going properly until I had done this tutorial about three times!

Hope this helps

Mark