It's not possible to have the external tanks quantity displayed on the mfd, because it hasn't been programmed for this.
It might be possible to activate a fuel selector (the standard one should work), but you would need an hot key (or some joystick buttons) for it, because by default there's no assignement. Look under the Controls section in FSX, and search for all the various Fuel Selector events, and assign a key/joy button to the one you need.
