Theres only ONE wire on the SUPPLY side of the ammeter, and it gets to the hot ungrounded battery post, often via wiring it to the big post on a starter switch/solenoid where the big battery cable attaches. The other LOAD terminal feeds hot battery voltage to Lights,,,,,Ignition,,,,,PLUS wires to the BAT terminal on a Cutout Relay or Voltage Regulator......i.e. the genny is on same side as loads like lights and/or ignition.
If an ammeter is wired at the correct polarity if you turn on lights or ignition just sitting not running, it should swing over towards - discharge.......if it does the opposite, swap leads side to side as its backwards.
My Troubleshooting Procedure will help diagnose the cause of non charging... Did you polarize the genny and try it??????????