you need to do something with your java installation. It is not difficult.
Your browser may not support Java or supports an outdated Java version.
It is possible that Java Support is either not installed or turned off.

For Internet Explorer users:
Select Tools/Internet Options..., Select Security / Custom Level
In the resulting list find Microsoft VM or Java VM. Select any value except 'Disable'.
If you can't see it, VM is not installed - you'd have to install Java VM.

Internet Explorer users can install Sun Java.
Sun Java Virtual Machine can be downloaded & installed from here.

Close and re-open your browser after Java installation. If you installed Sun Java make sure, that it is switched on -
In Internet Explorer go to Tools menu, - Manage Add-ons, - Enable or Disable Add-ons... and check, that Java Plug-in has "Enabled" status.

For Firefox users:
Install Sun Java Plug-in - it can be downloaded & installed from here.   Close and re-open your browser after Java installation.