It's probably likely that you can't unless there's a corporate T-Mobile outlet in the airport. If you're going to stay in Newark you'll need to find a T-Mobile corporate store and buy a starter kit there for $10. If you're in the US you can have a starter kit delivered to you for $7. Starter kits include the SIM and an activation code. Starter kits include 10 minutes of talk time. You'll need to purchase more time in the form of top-up cards, PINs, or use the *ADD (*233) to add money on the phone using a credit card. Unfortunately getting a SIM or getting a starter pack is not as simple or easy as it is in Europe or in the UK where you can pretty much get a starter pack at any corner store. To get a SIM with either national GSM carrier (AT&T or T-Mobile) you pretty much have to visit a corporate store to purchase a kit.