Press the left mouse button and drag to rotate the molecule about an axis fixed in the plane. The rotation axis runs through the centre of the picture and is at right angles to a line from the centre to the point where you pressed the button. Don't start too close to the centre of the picture or you'll find it hard to control the direction of the rotation axis. The circle of tick marks, at 15o intervals, should help.
To rotate the molecule about the axis perpendicular to the plane of the picture, hold the ALT key down, press the left mouse button, and drag around the centre of the picture. (On a Mac you should be able to hold the OPTION key down as you press and drag the mouse, but this doesn't currently work.)
Dragging with the SHIFT key down changes the size of the picture, and dragging with the CTRL key down moves the molecule across the screen. These don't work on a Mac either, as the operating system ignores the SHIFT and CTRL keys, but you probably won't need them.
Press the L key several times to cycle through different labelling schemes for the atoms. The S key cycles through different ways of rendering the atoms and bonds; if you have a slow computer you may find it helpful to use a less fancy rendering. The B key turns the bond display on and off, but this is unlikely to be useful.
Above the top left corner is a Reset button which will restore the molecule to its original position and orientation. The button next to it carries out the inversion operation on the current configuration, and the other three reflect in the XZ plane, the YZ plane and the XY plane respectively. The X axis runs horizontally along the screen, the Y axis vertically and the Z axis out towards you. They remain fixed as you rotate the molecule.
If you click the mouse on or near an atom, it will be highlighted. The highlight remains as you drag, and may help you to keep track of how far you have rotated the molecule. SHIFT-click removes the highlight (but probably not on a Mac). You can use the numerical atom labels for the same purpose.