I have an ESB running with its associated ActiveMQ instance. Now I want a system to be able to send messages onto the ESB using JMS. Question: Is the correct way for the client to post the messages onto the ActiveMQ (as found on tcp://61636 or whatever I configure in activemq.xml), and the let my ServiceMix BC listen for incoming messages on that queue, and then forwarding them onto the NMR for further processing?
Yes, what you describe above is very typical. Just configure the servicemix-jms component to listen to a queue and send messages on to another component.
Another way of doing this is perhaps to spawn a separate ActiveMQ instance, to which the ServiceMix BC connects a collects the messages a s the arrive from the external system.
Yes, this is possible as well. There are many, many possibilities with ActiveMQ.