Hi,
Well at fisrt you need learn BPMN 2. For learn jBPM 5 read all User guide, do some tutorials. The designer not support all BPMN2 components, but jBPM 5 support more components that is showed in the designer see this thread to more information.
For a well jBPM 5 reference guide we are waiting Salaboy book, hope he finish soon.
Juan