In order for the alt to start charging it needs to have power applied to the feild windings. After that it should start charging. The regulator will adjust the voltage to the windings to keep the voltage output within limits.
Check all connections and make sure they are clean. Also look to see if there are any fusable links or fuses that are burnt out. Check the plug at the alt with a test light to see if they are being powered up. If not the alt will not charge.
