Daniel Fuchs summarizes the resources available to people with questions about JMX technology. Foremost among these is the JMX-FORUM mailing list. There are over 1000 subscribers, so chances are someone will have an answer for your question! If your question is sensitive or you are shy, feel free to drop me a line, eamonn dot mcmanus at sun dot com.
The JMX plugin for NetBeans has graduated from version 0.x to version 1.0. It's now available from the main NetBeans Update Center, rather than the Update Center Beta as before. My colleague Jean-FranÃ§ois Denise (one of the driving forces behind this plugin) has the details on his blog.