Why wont it charge?

Have you checked the power inputs to the alternator yet?

The alternator needs a 12 volt supply to charge.

That 12 volts is on the yellow wire in the two-wire connector. It should be there all the time. If it's missing, look for a blown 15 amp fuse. Next, check if the "Battery" light on the dash lights up when you turn the ignition switch on. That circuit is the turn-on circuit for the voltage regulator. If the light is not on, check for voltage on the green / red wire in the two-wire plug near the generator or in the three-wire plug on the back of it.

If both of those are okay, be sure you have full battery voltage on the large output wire. That should also be there all the time.

If all the voltages are good, look on the back for the "Ground here to test" screw head. Jump that to ground with a piece of wire while the engine is running. Monitor the battery voltage or turn on the head lights so you can see if system voltage goes up when you do that.