Here is a series of incremental tests to see if ListenUp was installed correctly.
THESE TESTS MUST BE BROWSED FROM YOUR WEB SERVER, NOT FROM YOUR LOCAL HARD DRIVE!
If you are using Commons FileUpload jar to handle multipart form requests, then verify that Tomcat's class loader can access it. If your browser shows a page with a message on it (as opposed to a ServletException for example), then Tomcat is serving your webapp properly, and has found the Commons FileUpload jar file.
If this fails, you need to put the Commons FileUpload jar file in "WEB-INF/lib".
Test if you can upload a file from an HTML form to a JSP page. This HTML form mimics the ListenUp applet. If this works, the ListenUp applet will probably work!
You should examine <tomcatdir>/webapps/listenup_jsp/uploads on your server to verify that your file was uploaded ok.
More information on ListenUp can be found on the JavaSonics web site.