This I copied from a page on the sun site providing information about java to javascript communication, as of 1.3...
http://java.sun.com/products/plugin/1.3/docs/jsobject.html
To compile Java code to take advantage of JSObject, you must have the package netscape.javascript in the CLASSPATH. Currently, Java Plug-in 1.3 ships netscape.javascript in a JAR file called JAWS.JAR. To compile an applet which uses JSObject, please add JAWS.JAR in the CLASSPATH before compilation.
So it looks like you are looking for a jar file called JAWS.JAR. You may already have it. If you are on Wind, check the jre/lib folder of your jdk...
On Linux, check the jre/lib folder for a file called "javaplugin.jar" instead.