If you don't see a spinning Rubik's Cube or if you see a blank grey window
or an error message about an applet not loading, then you probably don't have the
latest version of Java, which you can download for free from here:
download Java
Also in the browser window, do Tools/Internet Options/Advanced and you should see
an entry like this - make sure it's ticked:

Once all that's done, restart your browser and try again.
Enjoy !!