The .zip extracts to a java applet (aka a program) that can only run in a java-enabled browser such as I.E 6.0 - you need to double-click on index.html to launch the program - as long as your browser is java-enabled there shouldn't be any other requirement.
The .class files contain the program code and can only be viewed if you a java IDE (Integrated Development Environment) program that allows you to write and compile Java programs - only needed if you wish to modify the program. If you do need one it can downloaded for free from the Borland site.
hth
Charles