The small documentation of Sweet Home 3D Viewer is in the README.TXT file provided in install file and the Javadoc of the com.eteks.sweethome3d.applet.SweetHome3DViewer class.

You can switch between Aerial view and Virtual visit modes in by simply pressing the space bar.
Next version will provide also navigation arrows by adding navigationPanel:"true" to applet parameters. These arrows should simplify the navigation for new users.
You may already test them in Sweet Home 3D 2.3 Beta version described here.