Hi Leo,
I think custom work item handler is attached at runtime on a process instance: you are not supposed to put it into Guvnor.
You just have to import into Guvnor process definition (BPMN file), maybe Drools files or Freemarker task screen definitions, but I don't think you should add custom work handler into Guvnor. That is something you do in your code right after you instantiate process instance.
Thanks,
Miljenko