private ImageButton imageButton;
 ....
 imageButton.addListener(new ClickListener() {
        public void clicked(InputEvent event, float x, float y){
            System.out.println("FCK YEAH!");
        }
    });

private Some some;//наследуется от актера
....
some.addListener(new ClickListener() {
        public void clicked(InputEvent event, float x, float y){

            System.out.println("CLICKED!");
            };
        }
    });