Please. Anybody help me!
What you're doing wrong? The user that is calling the EJB method is in the 'usuario' role and the EJB allows only users in the 'admin' role to call it. That's what the exception tells me.
It could be that the reason that you don't get so much response, is that it is a little hard to understand what you are doing - at least for me it is.
I don't know that cactus stuff, but i think your problem is not in cactus, is it? Why not write a simple testcase, e.g. using a standard servlet? It might help you find the bug, because it's a simple example, or you can post it here so we can really help you.
Oh, and btw: the fact that the code is in spanish (i guess ;-)) doesn't make it much more clear