Take a look at the section on domain-specific processes in the documentation:
https://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/jbpm-5.1-SNAPSHOT-docs-build/jbpm-docs/html/ch07.html
In short, you have to register a handler that will be used to execute your Email node.
In this case, you should make sure you have the jbpm-workitems jar on your classpath and add the following line after creating your session:
EmailWorkItemHandler handler = new EmailWorkItemHandler(host, port, userName, password);
ksession.getWorkItemManager().registerWorkItemHandler("Email", handler);
Kris