The theory behind it is that you send a talk signal from your "transmitter" radio to be received by the "receiver" radio. You need to come out of the headphone (listening) jack of the receiving radio and into the low-voltage trigger. The very low voltage (what would be going to headphones if the wire hadn't been cut and sent to the trigger instead) will cause the trigger to trip, thus allowing the power from the battery to go through and hit the actuator arm.
To troubleshoot, first try hooking the battery directly to the actuator arm. If the arm doesn't trip, then you have either a bad battery or a bad arm. If it does, then try hooking the battery to the arm, through the low-voltage trigger. Then apply a low-voltage audio signal to the trigger to cause it to trip. If the arm doesn't trip, and you have applied a measurable low-voltage signal (test with a multimeter), then you may have a bad trigger.