Hi Vincenzo, the new BPMN2 Editor for Eclipse is still in it's early stages and I would recommend to use the existing Rule-Flow editor which get's installed if you are using the jBPM installer and/or the jBPM Web Designer which also gets installer for you by the jBPM installer. The Eclipse Rule-Flow editor is currently the most stable followed by the web designer, but both feature-wise and stability-wise are currently better.
Hope this helps.