Persistence unit not found in seam-gen application - https://jira.jboss.org:443/jira/browse/JBDS-699
This is a common misconception. JBDS will pick up the persistence unit, but you have to refresh the project first. I've admitted that this sucks, but for right now it is just how it needs to be done. I will update the docs with a big old note.
-Dan