To select a particular class, simply click immediately above it in the heat map panel. The clickable part of a class is shown by blue rectangles in the following figure.

Selection of class in the heat map panel.

    You can also use the Left/Right keys on the keyboard to scan all classes. The selected class is always shown by an arrow (a red arrow in the Master Tab and a blue one in a Result Tab). The buttons above the arrow in the figure always refer to the selected class.