Everything is fully described in the documentation to be soon available on flashline (it should have been available yesterday...)
It needs to be set in the <cluster-config> element in the jboss.xml configuration file of your bean. 2 policies are available: first-available and round-robin. You can also easily implement your own policies (take a look at the two existing: they are only a few lines long).
Is there a way to set this for JMS queues?
There is no JMS-clustering features yet