ok, my mistake.
it doesn't work either with Sun App Server but just with bundled TomCat. So I guess this isn't really a jBoss/NetBeans question, but more a NetBeans conf question
NetBeans Http Monitor can be used probably with any J2EE server, the IDE, however, directly supports only Tomcat and Sun Application Server.
For more info refer to the NetBeans IDE help
Servers and Databases > HTTP Monitor > Setting Up the HTTP Monitor