I think this is the best explanation of NP that I have seen. Thank you!

There is some more information about how they trained the brain cells in this article: http://www.worldhealth.net/p/394,6110.html. From the article,

But how do the neurons learn how to fly the thing? That's done by electrical pulses into the dish through one of the electrodes. That in effect tells the neurons when they are doing the right thing to keep the plane on course. High frequency, or rapid pulses, stimulate the neurons and enhance the connections between them.

Simply put, by stimulating the neurons the researchers tell them they're on the right track, so they continue to adjust the plane's elevator to keep it from plunging toward the ground during a downdraft, for example. When the plane levels off, the simulator reduces the frequency of the pulses, and the neurons back off from that control surface, allowing the plane to remain on course.

After just a few minutes of that kind of training, the "brain" takes over completely, sending signals to the plane's control surfaces, and using feedback from the simulator to know just which signals to send.

As I understand it, one or more of the electrodes function as correctness feedback for the brain cells. These electrodes become more active when the simulated plane is flying level and in the right direction. More activation in these electrodes causes more activation among the brain cells, and when activation is higher the brain cells form stronger connections. So when the brain cells are doing what they are supposed to their behavior is reinforced.