Alright, found a solution for my problem. When using Guvnor to deploy processes (as I have been doing) it does not deploy your custom work handlers, to do so, I followed this:
Deployment to an appsersever (e.g., JBoss or Tomcat)
1.) Copy the configuration file (CustomWorkItemHandlers.conf) to gwt-console-server's META-INF directory
2.) Copy the Notification class to the gwt-console-server's CLASSPATH.
e.g., Copy the Notification jar file into gwt-console-server war file ( ..gwt-console-server\WEB-INF\lib)
or Copy the Notification jar and the dependant jar files into appserver's lib direcory (JBOSS_HOME\server\default\lib for JBOSS or TOMCAT_HOME\lib for TOMCAT)
From this post: