In order to monitor/manage custom MBeans using JON, you'll need to create a custom JON plugin that extends the base JMX plugin that ships with JON. There is an example of a simple custom JMX plugin checked into RHQ SVN here:
You can use that as a starting point. You will probably not even need to write any Java classes; when you extend the base JMX plugin, most things can be defined declaratively in the plugin descriptor.
For docs on writing custom plugins, see:
Let me know how you make out.