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.ExampleWebappComposer 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.
See downloads on how to download the struts2 module either using Maven or by downloading full distribution.