To connect to the Rainbow Server, you need to download public.jar and put it on your classpath when compiling and linking, and make sure that you've enabled dynamic loading of classes over the network, which is done by defining the property java.security.manager (or installing your own) and setting up your security policy correctly (either in your home directory or in a location you specify via the java.security.policy property) to permit the right set of actions. The one you'll probably want to add will allow the RMI stubs to contact the Rainbow Server, of which the following should be the most restrictive that you'll need:
grant codeBase "http://www.cs.man.ac.uk/~fellowsd/work/rainbow/rserver/export.jar" {
permission java.net.SocketPermission "athene.cs.man.ac.uk:1024-", "connect,resolve";
};
You probably want to grant your own code java.security.AllPermission (and you'll probably need to grant suitable file and network access if you don't so you can load files and contact the RMI Registry), but the public.jar file needs nothing.
Let me know if you've any problems.