I've created a screencast about how to create your own node types a while ago. You could try following that to see whether that works or what you might be doing differently:
http://people.redhat.com/kverlaen/DomainSpecificWorkItem.swf
Note that this screencast uses a new Drools project to get started, but it also works if you create a new jBPM5 project (I just verified to be sure).
Kris