How to upgrade ammo on black ops zombie map Five?

So i have turned on the power on the president level I upgrade my gun in the pack o punch adn then when I go to the wall to upgrade the ammo for 4500 it will not work. How am I supposed to do it?How to upgrade ammo on black ops zombie map Five?
That is for buying ammo for the upgraded weapon, not upgrading the ammo. To upgrade your weapon, you have to use the Pack-a-Punch machine. For the map Five, the machine is reached by turning on the power, then pressing the four switches scattered around the war room (the one with the upper catwalk and lower level with all of the maps and computers everywhere). Go through the teleport showing the machine to go to the correct room. Note that after a time, the machine will spin behind the wall and vanish again. Then you will have to reset those four switches again.

Another way to get to the machine is to get the Bonfire Sale which is gotten by killing the Pentagon Thief before anyone has their weapon stolen. Again, only happens on Five.

Edit: Plus, you seemed to be pretty aware of the Pack-a-Punch machine when you asked your previous question I answered.