if you are using jbpm installer look in the $jbpm_installer/conf/META-INF directory. there you will find a file called CustomWorkItemHandlers.conf. You would need to register your workitem in this file and run
ant clean.demo
ant install.demo
for it to get copied to the right location. Otherwise you would need to manually copy CustomWorkItemHandler.conf and drools.session.conf into
jbpm-gwt-console-server-war/WEB-INF/classes/META-INF/
Hope this helps.