Are you creating your process using some kind of IDE?
Try using a <Task> node instead of a <ServiceTask>. Something like:
<task id="_11" name="Multiply" tns:taskName="myTaskName">...
Then, in your code you can do:
ksession.getWorkItemManager().registerWorkItemHandler("myTaskName", new MyTaskNameWorkItemHandler());
You can also try to use 'tns:taskName' attribute in your <ServiceTask> node to see if it works there too. Please let me know the results.