You need to add a work item handler for the task.
eg. if it is a human task, WorkItemHandler is added like this :
ksession.getWorkItemManager().registerWorkItemHandler("Human Task", new WSHumanTaskHandler());
if it is mail node, then WorkItemHandler is defined like this :
ksession.getWorkItemManager().registerWorkItemHandler("Email", new SystemOutWorkItemHandler());
regards.