wishmaster7,
I am pretty sure it has nothing to do with JBPM 5.4 combined with PostgreSQL as that is exactly what I am doing. (I am using postgres 9.1). You might try dropping the web app example I created into your jboss installation and see if it connects. It does not depend on anything except the drivers and has instructions on how to set those up as well. If you can drop my web app into your installation, and it works, than you know it's some sort of configuration in your setup.
HTH