From the little I can undertand of your message, I guess you are using either the PMDG 737 or maybe another of the airplanes that have an APU recognized by GSX, so Pushback won't start until you turn the APU on.
Are you sure you want to use such complex airplanes without knowing how to turn their APU on/off ? Refer to their original documentation, I'm fairly sure they explain how to operate it.