The error that you are getting is because you have the jar file (jdom-1.0.jar) in your IDE but not in guvnor's classpath. You should add that lib to the server in order to be able to deploy that process.
By the way, you can use service tasks instead of script tasks and create a class that has the dependency on jdom, instead of bundling java code inside the xml process definition. The newly integrated service repository will allow you to download the dependencies as well, so you will not need to download and place the jar inside the container by hand.
Cheers