It would be useful if you could paste the exception you're actually getting.
But looking at your description, you're using your own classes etc. so you should
(1) make sure that these classes are available on the classpath, so drop them in a jar in the server/default/lib folder
(2) register the handlers for each of the service tasks you're using
I just added a new part to the documentation on how to add your handlers in the jbpm-console:
http://docs.jboss.org/jbpm/v5.1/userguide/ch11.html#d0e3081
In this case, you should register
"Service Task" : new org.jbpm.bpmn2.handler.ServiceTaskHandler()
If you still get exceptions, it would be great if you paste a more detailed exception.
Kris