If you are new to SiteMesh browse over to Getting Started with SiteMesh which covers the basics and how to setup SiteMesh in your web application.
SiteMesh build and source code are available at GitHub. The jar files are also located on the Maven Repository.
SiteMesh 3 is in the works. Please try it out and provide feedback.
sitemesh-example.war - a pre-packaged web project with many examples of SiteMesh in action.