if you know how to use JBoss AS 5.1 and Eclipse 3.5, the only thing that you need to learn is jBPM5.
Did you try to use the installer provided in the download page?
It works with jboss 4.2.3 I think, but if you know a little bit about JBoss AS 5.1 you can just move the deployed files and configurations to that version.
Greetings.