I've forgotten, I use a MySQL version 5 and I'm a novice in JBoss and JBoss Portal
I can give you more precision if you need.
To use JBoss Portal with MySQL, you will need to setup you database connections so that the portal container can setup all the tables that it needs to function.
You should be able to take the following steps (give or take a couple) to get things working. (Portal_home is the root directory where the portal is extracted)
1. Go to the [Portal_Home]/setup. Copy the file portal-mysql5-ds.xml to [Portal_Home]/server/[config]/deploy directory. Where [config] is the server configuration that you chose... all, default, minimal etc.
2. Edit this file at it's new location to include your database server's connection URL and everything else that is needed to connect to your database server.
3.Go to the MySQL site to download a copy of the JDBC driver for MySQL and place the JAR file at [Portal_Home]/server/[config]/lib. I think it is available here: http://dev.mysql.com/downloads/connector/j/5.0.html
4. Startup your server and you should be good to go.
Hope this helps a bit.