More info: Mac OS
Macintosh users cannot use Netscape Communicator 4.72 and below due to its pathetic support for Java on the Mac; instead, use Internet Explorer or the beta versions of iCab (a new Mac-only browser) or Netscape 6.0
Users of Mac OS 8.1 and below must download Macintosh Runtime for Java (MRJ). Version 2.2 is currently available and works quite nicely, thank you. Mac OS 8.5 and higher installs a new-enough version of MRJ and can bypass this step.
Finally, all Mac users must install the Java Swing libraries. First, download it, then drag the swingall.jar file to the following location:
Startup Drive: System Folder: Extensions: MRJ Libraries: MRJClasses
Now restart your machine.