Thanks for the response,
Well the user guide recommends installing the plugin from the following update site:
http://codehoop.com/bpmn2
And here's the section in the guide that recommends the plugin (maybe someone should write a note in there to point people to the old plugin):
http://docs.jboss.org/jbpm/v5.1/userguide/ch09.html#d0e2936
So basically the guide recommends the new aqua-blueish designer as in the following picture:
http://docs.jboss.org/jbpm/v5.1/userguide/images/Chapter-Eclipse/EclipseJBPM.png
Other parts of the guide (e.g. human tasks) are still using the old plugin however.
Btw, in the guide it is mentioned that you would provide a profile for the eclipse plugin that limits the available constructs of the designer to the subset supported by the engine. I couldn't find that profile, so all elements are available for me right now, and I can't figure out which ones are supported.