You should download and install the jbpm plugin that you mentioned, as that allows you to create a simple jBPM5 project, create a runtime, etc. At this point, we would recommend you use the Drools Flow plugin for editing your BPMN2 files though (as the new jBPM5 BPMN2 editor isn't ready yet). So this means you just install that plugin alongside your jbpm plugin so that the Drools Flow editor will be used when you try to open a *.bpmn file. If you use the installer, it can generate a fully functional Eclipse environment for you to get started.
If you still have issues (like the one you mentioned above, which should be solved if you define your *.bpmn process inside a jBPMN5 project), please give me a concrete example of what might be failing and I'll look into it. You can find numerous examples here as well:
https://github.com/krisv/jbpm/tree/master/jbpm-bpmn2/src/test/resources
Kris