Detailed description of this issue can be found in the following discussion thread.
I am attaching a patch that updates the struts2-sitemesh plugin to use the more current SiteMeshPageFilter.java class. The old code was deprecated and deleted from the sitemesh with the 2.4.2 release!
The new files are as follows. I am testing the freemarker extensively, and will test the velocity a bit later. Please test both and let me know if there are any problems.
FreeMarkerMapper2DecoratorSelector.java OldDecorator2NewStrutsDecorator.java VelocityMapper2DecoratorSelector.java
FreeMarkerPageFilter.java OldDecorator2NewStrutsFreemarkerDecorator.java VelocityPageFilter.java