You need to make sure that all actors that you want to assign tasks to are defined in the human task service. The evaluation example assigns the initial evaluation task to the user you fill in the form when starting the process. You should make sure that that name is known. You can configure the users here: https://github.com/krisv/jbpm/blob/master/jbpm-installer/task-service/resources/org/jbpm/LoadUsers.mvel
So either use one of the known users (the demo suggests to use 'krisv') or simply extend this file on your installer to include additional users.
Kris