So, trying to draw a bit of a mental map of LIM here.
Go through check in, and exit through the left side. If you're on the right side, go all the way to the left and there'll be an exit to the check-in area there.
Go up the stairs/escalator. Then, take a right.
Keep on walking, until you see a walkway to your left. Take it, and just as you turn, the ATMs will be there, to the right. Of all Peruvian banks.
They're just next to Mr. Pretzels, and in front of Pikeos.
Hope it helps!