Which Jboss are you using? Under 2.4.3 the JAAS stuff is there by default... then you add your login configuration stuff to auth.conf.
See http://www.jboss.org/online-manual/HTML/ch13s84.html for details. (I believe this is where you got the instructions to add the mbean. do a search and see if it is already enabled in the jcml file. also make sure you do not have a jboss-auto.jcml (or something close to that.. I cant remember the exact name :)
I have posted a couple times in here on how to get the database login to work so you might have a look at those.
Thank you foglesa