As this image indicates, you must click on the either of the two right-side arming switches, for the tanks to load. Some variants have 75-gallon tanks, some have 108-gallon tanks, and some have no tanks at all. They can be released (in an emergency of course!) by clicking on the "Train Bombs" switch, also pictured, and then either clicking on the top control stick button, to release both tanks, or by clicking on the individual, manual release levers, below the engine control unit, which will drop the tanks one at a time.
By default, just as in the restored aircraft, you only have the option to fill the left and right main wing tanks. The fuselage tank and wing tanks, if in-place, are really just for display, even if functional. However, both 75-gallon and 108-gallon external fuel tank entries are provided for within the aircraft.cfg file. Simply remove the // marks from one of the sets of external tanks, and then you can fill them with fuel in the sim (activating both 75 and 108 gallon tanks really won't work, nor is it accurate).
[fuel]
Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
fuel_type = 1
number_of_tank_selectors = 1
electric_pump=1
LeftMain = 0, -4.37, -2.28, 92, 0
RightMain = 0, 4.37, -2.28, 92, 0
//Center1 = -6.5, 0.0, 2.620, 85, 0 <- 85-Gallon Fuselage Fuel Tank
//External1 = 0, -7.51, -3.23, 108, 0 <- Left 108-Gallon Drop Tank
//External2 = 0, 7.51, -3.23, 108, 0 <- Right 108-Gallon Drop Tank
//External1 = 0, -7.51, -3.23, 75, 0 <- Left 75-Gallon Drop Tank
//External2 = 0, 7.51, -3.23, 75, 0 <- Right 75-Gallon Drop Tank