Sorry, i forgot to mention two things: i'm working with jpdl 3.1.7 and with jbpm-simulation, so i don't only have a processdefintion, i also have a experiment-definition.
i found http://docs.jboss.com/jbpm/v3/userguide/tutorial.html#taskassignmentexample this example and it is said that i'm supposed to add a few lines of code, but since everything in my project looks a bit different, this example doesn't work for me.