There are two fundamental reasons why Java might not work:

1. The Java Virtual Machine may not be installed.  A version of this used to be included with Microsoft Internet Explorer but lately it has been withdrawn.  You can get a copy of it here.  The proper version is now supplied by Sun and can be downloaded here

2. Security options in your browser may be preventing Java from operating.  If you are using Internet Explorer, go to Tools/Internet Options/Security and set some custom settings.  In principle, wherever Java is mentioned, allow it!

Meanwhile, try this link