Sorry for what must be a really stupid question but after having placed the MSFS Addon GNS530 in my Community folder, I can't seem to turn on the unit from inside the aircraft.

You are referring to the PMS 530 found at https://github.com/pimarc/pms50-gns530 correct? Which aircraft are you trying to get it to turn on in? If bat, alt and avinoics in the plane are on, then try turning the C volume knob (right below the radio freq switch knob on the upper left) up. If you look close at that C knob, it says PWR/VOL for rotating and PUSH/SQ for clicking. You want to make sure it's rotated on.

Many thanks. Looks like I downloaded an incomplete file. Your link fixed it.  ;o)