The official JBoss Messaging project page is http://labs.jboss.com/jbossmessaging/.
The download location is the JBoss Labs Messaging Project download zone: http://labs.jboss.com/jbossmessaging/downloads
If you want to experiment with the latest developments you may checkout the latest code from the Messaging SVN trunk. Be aware that the information provided in this manual might then not be accurate. For the latest instructions on how to check out and build source code, please go to Messaging Development wiki page, specifically "Building and Running JBoss Messaging" section.