Java may not be installed correctly.
Check to make sure you see a Java coffee cup icon on the top left corner of the gray program screen after clicking the link to start the program.
If this icon does not appear, re-install Java.
On the bottom left corner of the browser, you should also see either "Applet RoboSensei loaded" or "Loading Java Applet ..." while waiting for the program to appear. Depending on the computer, it may take a while (more than a minute) for the program to appear on the screen.
If you don't see the icon and either of the above messages, you don't have the Java plug-in installed correctly.