Turn on the Click to Select Elements option. In the image subwindow, drag a rectangle by holding down the mouse button as you move the mouse. The selected elements should change color. A pull-down menu of different statistics types appears at the bottom of the Menu. Use the function selected to gather statistics on the selected elements.
Output appears in the message window. .
You might want to play with various menus and see what's available. When you want to quit, go back to the main OOF Menu and click on the quit option in the Function list, or the Quit button at the top of the window.