Hiya Lilbond. It wasnt a guide to help find planes, just to ID them. They are found at all longitudes and latitudes. If you download the placemark in the first post of the following thread youll see tons of them in flight that were found around the globe. All Planes In Flight
As far as where to look, I found that the most popular airports have little planes. I dont know why. Take Logan Airport in Boston. A VERY busy airport with alot of activity on the runways, yet only 1 or 2 planes found in flight in the entire state of Massachusetts. I dont believe too many are from the Chicago area either. The most planes in the US (i think) are in California. Probably because the state is so large and there are so many data chunks there. But also because its a sunny warm popular place to fly year round. Not to mention the constant flow of tourists.