Hi megas,
and welcome to our forum
I'm not sure if I understand you correct: your jOpenSim can not connect to the OpenSim database? Where do you use what wildcards?
Mostly the reason can be a missing user for the opensim database in the database server of OpenSim with the hostname of Joomla. E.g. if you use opensim@localhost inside your OpenSim server, you will need a duplicated user opensim@host-of-your-joomla to give it access to the database.
Another reason for connection failure might be limitations in the configuration of MySQL on the opensim database server side. Something like "skip-networking" or "bind 127.0.0.1" will make MySQL refusing any connections from outside.
Also to check is if the MySQL port (3306 by default) is opened for the Joomla server by the firewall of the OpenSim server.
I hope this helps
Greetz
FoTo50