This seems to be a problem with your Java installation; the class that is not found should be in the jdk/jre libs.

A fix that should work:
a) check that you are using java 1.3 or above
b) find the path to a file called "rt.jar" , usually located in /path/to/your/Java/Install/jre/lib/rt.jar
c) edit the start script: append the full pathname of rt.jar to the CLASSPATH variable that is set there.