I'm not with Laminar Research, just a fellow simmer in the UK.
It's going to be difficult to provide advice without knowing the specifics, but in XP11, all aircraft will be in a 'dark and cold state' when loaded into XP11 (i.e. no electrical power available), unless you have ticked the 'Start with Engines Running' check box when loading the aircraft to be flown. You will find this checkbox if you click on the 'Customize' button for the aircraft in question.
If you do not tick the 'Start with Engines Running' checkbox, then you will have to start the aircraft from a 'cold and dark state' manually, once the plane has loaded into XP11.
If required, you will need to have a ground power unit (GPU) attached and switched on, although some GA aircraft don't actually need this as they start up from onboard battery power.
Either way, you will then need to switch on the relevant battery and avionics controls to supply power to the instruments and also to enable you to switch on the engines as required.
Hope this helps initially, but please ask if you require any further information?