Luis,
The workitems handlers are mapped to "task types" and not task names. If you are using the custom Email node which comes with the pallete, you should register it with "Email". Otherwise, you will have to register with "tns:taskName" value which you can find it if you open the bpmn as xml.
Usually, these are custom nodes and you define the "type" in the .conf with your nodes, and there you will find the name of the node type (http://docs.jboss.org/jbpm/v5.1/userguide/ch13.html#d0e3540)
Demian