LiferayIPC error message when I deployed it as a liferay portlet

I tried the LiferayIPC add-on and deployed the two sample portalets (one sender portlet and one receive portlet). I got following error:

Widgetset does not contain implementation for org.vaadin.artur.liferayipc.LiferayIPC.
Check its @ClientWidget mapping, widgetsets GWT module description file and re-compile your widgetset.
In case you have downloaded a vaadin add-on package, you might want to refer to add-on instructions.
Unrendered UIDL: org.vaadin.artur.liferayipc.LiferayIPC(NO CLIENT IMPLEMENTATION FOUND)

Any idea what might be the problem? Many thanks.