Assuming that's a custom class - is the class or jar file on the classpath?If the file is being built within the IWHOME/local/config/lib/content_center/customer_src/ tree with make_toolkit.ipl - then it should be. Is that not how you're doing things?
implemented in both ways keeping in customer_src/src folder and also creating jar, placing in customer_src/lib
but observed one thing there is sample class in workflowmodelerapi.jar (InPostProcessDriver class in package com.interwoven.modeler.workflow.commands) which is getting loaded and i had also copied same code and even tried it.