First leave a crawler somewhere, so you have time to do everything required.
MAKE A TURBINE
Go to the bus station and make a turbine.
To do this you need to get a mannequin from the central map, get the wing of a toy plane from the payphones, and also a fan from behind the workbench.
If you put all these items on the workbench one at a time, then you'll create the turbine.
Once you have your turbine, grab it and leave the bus station. Get on the bus and take it to the Power Station.
To get inside the Power Station you will need to buy your way through the door directly across from the bus stop.
Once inside, turn on the power by collecting the Circuit Board, Lever, and Zombie Arm which are all sitting around the power station.
Attach those items to the power switch and hit it to get the juice flowing.
Once the power is on, place the turbine you are carrying near the door with the lightning bolt on it. This should open it. Note that if the turbine shorts out you'll need to get another one, or get a friend with one to help you out.
From here you need to proceed to the bank. Unlock the front door and use a grenade or the ray gun to blow the two vault doors.
Once inside you'll see another door with a lightning bolt on it. Provided that your turbine is still working and you put it in the right spot, the door should open.
MAKE THE PACK-A-PUNCH
Go through the door into the bunker and grab the Battery, Shell and Stool, which are all lying around the place.
Once you've gathered those items, take them to the workbench and construct the Pack-A-Punch machine!
Now you can buy powerups for your weapons for 5,000 points each.
[more cheats for Call of Duty: Black Ops 2 here]