If you experience problems with using add-ons, you can try the following:
Check the .gwt.xml descriptor file under the the project root package. For example, if the project root package is com.example.myproject, the widget set definition file is typically at com/example/project/AppWidgetset.gwt.xml. The location is not fixed and it can be elsewhere, as long as references to it match. See "Client-Side Module Descriptor" for details on the contents of the client-side module descriptor, which is used to define a widget set.
Check the WEB-INF/web.xml deployment descriptor and see that the servlet for your UI has a widget set parameter, such as the following:
<init-param> <description>UI widgetset</description> <param-name>widgetset</param-name> <param-value>com.example.project.AppWidgetSet</param-value> </init-param>
Check that the widget set class corresponds with the .gwt.xml file in the source tree.
See the VAADIN/widgetsets directory and check that the widget set appears there. You can remove it and recompile the widget set to see that the compilation works properly.
Use the Net tab in Firebug to check that the widget set (and theme) is loaded properly.
Use the ?debug parameter for the application to open the debug window and check if there is any version conflict between the widget set and the Vaadin library, or the themes. See "Debug Mode and Window" for details.
Refresh and recompile the project. In Eclipse, select the project and press F5, stop the server, clean the server temporary directories, and restart it.
Check the Error Log view in Eclipse (or in the IDE you use).