Oh it's an essential item for any trip to the USA
http://www.macys.com/store/about/visitor/index.jsp
Oh, it has dropped to 10% I have just noticed, it used to be 11%
Anyway you go, usually to the gift wrap counter that is on one of the upper floors and you just ask for it. I was asked for a passport once, but most times I have just had to show my hotel room key and obviously my accent is enough.
US residents travelling get one that is valid for 5 days, I used to get those from the concierge desk in the hotel we stayed at in Hawaii, so it saved trying to find the counter in the store. So it's possible that they concierge at your hotel in NY may keep them, but just the 5 day ones. Otherwise grab the 30 day one in the store.
You get 10% off everything in the store except cosmetics and perfume. So if there is a sale rack with 25% off or 50% off, then you can still use the voucher for another 10% off. I once bought a pair of Tommy Hilfiger jeans, they were on a 50% off rack, there was 25% off storewide and I used the 10% voucher. I got 85% off, I paid $11 for them