An abstract class cannot be instantiated. You must create a "concrete" or real class which implements the abstract class, which you have done.
The abstract class may also define abstract methods which the concrete class must supply. In this case, you have not supplied a method for onPresentScreen(), hence the error you are seeing.
http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
See the AdListener documentation for how to implement this method which is called when an ad is presented at full screen.
@Override
public void onPresentScreen() {
// called when a full screen ad is presented
}