In order for the applet to run, it is necessary to check whether the Java plugin is installed in the browser and whether it is enabled. Depending on the browser, this can be checked in:

Mozilla Firefox Open Menu -> Add-ons -> Plugins. You should be able to see the "Java(TM) Platform" plug-in and on the right hand side it should say "Always enable".

Internet Explorer: Tools -> Manage Add-ons. You should be able to see "Java(tm) Plug-In SSV Helper" and "Java(tm) Plug-In 2 SSV Helper" and that they appear as "Enabled".