If you are seeing an exception like the following with a CMP bean on JBoss:
javax.naming.CommunicationException [Root exception is java.lang.ClassNotFoundException: org.jboss.proxy.ClientContainer (no security manager: RMI class loader disabled)]
You need to navigate to your Window > Preferneces > MyEclipse > App Server > JBoss 3 > JDK panel, and add this under Optional JVM Arguments:
-Djava.security.manager -Djava.security.policy="server.policy"
and then add the policy file in your Eclipse directory. If you run into trouble, please see this thread.