You will first need to turn the power on in the map by activating the following power switches:
U-Boat Pens - near the Ripsaw buildable;
Artillery Bunker - near the Faustblitz (melee perk) machine.
Once both switches are activated, you will need to ride the minecart from 3 different locations.
While on the ride you will need to pick up
3 Elektroschnalles by pressing the
button.
You must take the following rides:
Route 1: Beach mine cart to U-Boat Pens;
Route 2: U-Boat Pens mine cart to Bunker 1;
Route 3: Bunker 1 mine cart to Beach.
When you collect all 3 Elektroschnalles, head outside near Bunker 3 and insert the 3 batteries into the device in front of the elevator.
Once all are placed the Pack-A-Punch machine will rise up form the now powered elevator.
Video Guide: