Things are slightly different with Netscape 6.0. No longer do you get a Java VM packed and embedded in the browser. Instead, it depends on the one you have installed in the OS. If you don't have one installed or if Netscape can't find it, then Java won't work. There's not a great deal of help provided on this aspect of the installation.
Once you do have it installed and working, the Java connectivity should behave as it did before.
Writing a Java applet to handle a rollover effect is not such a challenging task as you might think, and is one of the things that can be accomplished quite early in your career as a Java programmer. Java is capable of much more than that, but if you are going to tackle a large Java-driven project, you need to think it through and plan it much more carefully.