You have to add it to the 20%, 50%, or 100% EMS screen where you want to see it, so go to the EMS screen layout editor, the size you want, press "INFO" and choose "FUEL EFFICIENCY" and then place it where you want it.
Fuel economy needs only two sets of data: Fuel flow and GPS. Fuel level calibration doesn't matter.
If you don't have fuel flow working, then MPG won't work. If you don't have a GPS (including no GPS lock) then it will also not work. Are you in a hangar with no GPS lock? Does your fuel flow show a red X?