By default JBoss saves dead letters in the DLQ. With Hypersonic as your persistence provider that means that the message can be found in the internal Hypersonic DB in the JMS_MESSAGES table. If you changed your persistence provider or even your JMS provider it may depend on that implementation where dead letters get stored. This stuff is configured in the proxy-invoker, see standardjboss.xml.
Could you please tell me how to listen the topic deployed on ActiveMQ ( Or Foreign MQ Proviers) from MDB deployed on JBOSS?. I am using JBOSS 4.0.2 with MDB ( ejb2.0). Please tell me what the configuration files i need to chnage. If you send me snippets would appreciate.