You must ZIP the extension class files according to the rules of a Java archive file. To run the Control Center tools as an application, the ZIP file (db2plug.zip) must be in the classpath. To run the Control Center tools as an applet, the ZIP file must be located where the <codebase> tag points to in the Control Center html file.
The ZIP file should be built will no compression and maintain the relative path positions of all the class files (zip -r0 db2plug.zip *.class).