I have exactly the same problem when trying to add the Hello World example.
I download the demo package, install it (ant install.demo), start it (ant start.demo) and then in eclipse I create the hello world example, I run it as a Java and in the console runs ok.
Then, I transfer it to the Guvnor repository. And when there I try to build it, it gives me exactly the same problem.
We are trying to evaluate if we use it for our company. Can you guys provide a simple, simple example where you can build from scratch a simple Hello World, that runs on eclipse, it is transferable to guvnor, you can modify it there, and later exercise it with the jbpm console....? This will be really valuable. I am eager to show my management that we can use jbpm, but with such documentation ...