The JavaScriptLab.java is using the LiveConnect capabilities of Navigator. It therefore imports netscape.javascript.*
These class files are found in the java_30 file that is normally found in the folder Netscape/Navigator/Program/Java/classes. This is an uncompressed zip file for Navigator 3.0. You may need to unzip these files and place the resulting directory somewhere in your classpath setting to allow the javac compiler to find the necessary netscape.javascript.* files.
If the compiler cannot find the proper files you will usually get the following error message:
JavaScriptLab.java(7): Package netscape.javascript not found in import
Check you classpath and make sure there is a directory ./netscape/javascript somewhere in there. Also remember that the AppletViewer does not support JavaScript and cannot run this Lab.
See the Netscape home page for more information about LiveConnect and JavaScript.