It seems to me the UML module of NetBeans is a bit too much hidden. In NetBeans 6.5 it was very easy to create an UML diagram. No plugin installation necessary or sth. like. Read my post where I found a zip file to install the UML module.
And now, after this procedure, I got the UML module back, but it seems to me that I cannot create class diagram with it. Do you know how I can do this with NetBeans 6.8?
Update1: There seems to be no support
Update2: Nevertheless somebody seems to got it working.
Converted to answer: The UML plugin is no longer supported in new versions of NetBeans as stated in the wiki http://wiki.netbeans.org/UML. Not sure why, but that seems to be the case, does not answer your question, but it does put it in context. I could be just slowly breaking which is why both you and the blogger you linked to can't gets things to work.