To configure PicoContainer to handle the dependency injection of Struts2 actions, refer to the web.xml below. For a start there is a slightly different listener to the one defined in the main page. Also, instead of using Struts2's servlet, use ours. Everything else from the standard Struts2 setup is appropriate.
Struts 2 Webapp webapp-composer-class org.picocontainer.web.sample.ExampleStruts2Composer picoFilter org.picocontainer.web.struts2.PicoObjectFactory$ServletFilter strutsFilter org.apache.struts2.dispatcher.FilterDispatcher picoFilter /* strutsFilter /* org.picocontainer.web.struts2.Struts2PicoServletContainerListener index.html
The Maven example project is here. Make sure you make and configure your own WebappComposer implementation. Note that we use a base class for Struts2 applications: Struts2Composer
See downloads on how to download the struts2 module either using Maven or by downloading full distribution.