How do I connect the wires to this USB receptacle?
You didn't say how the old outlet was wired, but I'm assuming that both whites were connected to the screws one one side and both blacks on the other side. In this case, the tho white wires were effectively connected together and the two black wires were also effectively connected together.

Put the three black wires(two from the wall and one from the outlet) together in one wirenut and put the three white wires together in another wirenut.

Note that if you are using small wires nuts provided with the new outlet, you may need to buy larger ones to fit all three wires.