You must add the class file that you want to the server classpath. E.g.: CLASSPATH=%CLASSPATH%;%YOUR_PATH%\sample\lib, where the lib directory contains the entire package path to the desired class file (eventually a jar file).
This is a known issue in BEA Weblogic 8.1 and is not solved in SP2.
Check the "Bea Weblogic Server and Weblogic Express Release notes", page 94, CR106305.