Step 2, it's enough to place it in jbpm-gwt-console-server.war\WEB-INF\lib
Step 3 is not really needed for runtime.
where is this file (CustomWorkItemHandlers.conf) located? If you look at 5.4 documentation and the link I placed previously you'll notice that there is something called session.template that can be used to configure handlers as well.
In server.log you should see more details about the error.
HTH