To run the applet, you may need the latest Java Plug-in for your browser. It may offer to download it for you when you open the applet. Otherwise, you can got to the Java Plug-in page. To control the interface (assuming a 3-button mouse):

Left-click (and drag) moves around the plane. Right-click (and drag) zooms in and out--dragging left zooms in, right zooms out. Middle-click and drag to move points in the graph.

For an example of how you can use this applet yourself, view the source of this page. The PARAM tag enables you to feed your own initial nodes and edges into the applet to load the graph when it starts. Embed the APPLET tag in your own web page to use this applet for your own application.
Note: The version of Zoomgraph in the applet is somewhat old and not fully-featured. To get all of the really cool features of Zoomgraph, please download it and run it as a Java application.