I meant the jboss-ejb3-all.jar, of course;-)
You need to install JBoss AS using the JEMS installer (or update the EJB3 libs of your installed AS with libs from EJB3 RC8 but previous oprion is easier)
See the "Getting started" doc http://labs.jboss.com/portal/jbossseam/gettingstarted
Thank you, Thomas ! I reinstalled jboss.