If you really want to use web designer then you should use it through guvnor (using the URL posted by Marcelo). In guvnor, every time you create or edit a BPMN2 process the web-designer is going to be invoked.
Now, if you only want to check whether the designer deployed correctly or not you could use this url: http://localhost:8080/designer/editor?profile=jbpm&uuid=