it's kind of in the middle of the rental car desks. I always turn left when I get off the stairs/escalator. There are usually a bunch of signs for it, and a desk with someone sitting there to help you work the machine if you don't know what you're doing.
ETA: I see that they've added things for them into the other terminals now. We have only used it when it was only available on A side. I would guess that it's about the same for all of them though.