Should be possible to get around that by mapping a keypress or joystick button to turn on the alternator. Chances are they just forgot to put the button on the panel.
You can verify whether it's working by checking the voltmeter or ammeter gauge if there is one, or by using x-plane's Data Input & Output screen to dump the voltage to the screen. If the voltage is dropping after engine start, then you're in trouble. If you have landing lights, nav lights and strobes on, it'll drain even faster. That's all built-in to X-Plane already (assuming the aircraft designers have entered reasonable current draws for all the components).
Once you get to about 75% of the nominal bus voltage, kiss your radio goodbye. Yes, I had a lot of fun writing that
