Okay, I found the answer at https://community.jboss.org/message/506014.
I downloaded and installed the stand-alone runtime, let the plugin point to its location and finally I'm able to create processes and deploy them on the runtime in ESB (although it seems kind of an ugly way to do it).
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Well that is because ESB doesn't bundle the full jbpm runtime distro, only the jbpm jars.
If you had used SOA-P it would have been bundled and if you used JBDS it would have been configured automagically