There are two display windows appearing in the upper-left and lower right corners. The upper-left display window shows the rendering based on your model and rendering choices. The lower-left window initially dislays the grid of a two-dimensional coordinate system. The second window is used in selecting control points as you create a spline.
The window has two interaction controls attached. The first allows you to rotate the model about the x and y axes. This is accomplished with dragging the mouse with the left button pressed. Horizontal movement of the mouse rotates the geometry about the y-axis while vertical movement rotates it about he x-axis.
The second method of interaction is with the up and down arrow keys while the mouse is inside the window. The interaction is applicable for any of the models except Spline Only. In the case of Motion Path, each key press moves the sphere to the "next point" (up-arrow) or the "previous point" (down-arrow) in the spline. When the model is extrusion or surface of revolution, a press of the up-arrow key will add another quad-strip to the model while a press of the down-arrow will remove the last strip added.
As noted, this window is for creating and/or selecting control points. When the interaction mode is Add Control Point, a left mouse click will append a point at the mouse position as the last control point of the current curve.
If the interaction mode is Insert Control Point, a left click will create at the mouse position a control point between the nearest two.
Last, when the interaction mode is Select Control Point, a press of the left mouse button over a existent control point selects that point. While the button is down, you can move the selected control point by dragging the mouse.