I am trying to map a complex result to a process variable with the jbpm plugin for eclipse. But I can only choose between simple types (integer etc.) and "undefined". How can I set the type to my complex type?

I looked into the Simple-Workitem-Sample and found the following lines:

<itemDefinition id="_multiplierOperationItem" structureRef="org.plugtree.training.jbpm.model.MultiplierOperation" />

<property id="multiplierOperation" itemSubjectRef="_multiplierOperationItem"/>

How can I define an itemDefintion and set the structureRef to package.my.ComplexType with the editor, so that I am able to choose the ComplexType from the variable type dropdown list?