Where is the avionics master switch? :-?

There isn't one in most modern airliners.  It is a tied and computerised function to the electrical busses.  On battery power, avionics are on a needs basis (only really necessary avionics receive power), once a more reliable source is provided (APU, for example), all avionics should be on and ready, save those that are dependant on the IRS/FMC.

In short.  It's not a switch you operate as a pilot, so you don't need to look for it.

Happy flying,

Kyle

Thanks :-[ I've intsalled Ernie Alston's FMS but it doesn't come on when the APU is started :(

if to key over the blank area, just left of the autopilot controls, and left click the radio obs, and frequency box will come alive, hopefully with your version also. ::)