Instructions
Start the game and play as you normally would until you find the perk machines you want. In every match, the individual machines can spawn in a different hut on the map, so open the doors to as many huts as you need.
Walk up to the machines with the perks you want and press and hold the "X" button (square button on PS3) to purchase them. When you walk far enough away from the machines they will now disappear.
Continue playing as you normally would until you get knocked down by a zombie. When you get knocked down you lose all your perks, so it is now necessary to revive the perk machine(s).
Walk back into the hut that previously held the perk machine and stand right in front of where it was. After several seconds a perk machine will reappear in the spot. Repeat Step Two to purchase your perks again.