how to use vaadin in a google app engine application

I am trying to create the UI for a Google App Engine for Java web app

(1) I tried creating a new Vaadin project- but when I try to run it I always get an error- Google.AppEngine.Datastore class is not recognized. How do I fix this error

(2) I would like to use Vaadin in an existing Google App Engine web application- how do I do this without going through the new Vaadin Project Wizard?