[Newbie] DataSource configuration under Tomcat

[Luis Fernando Planella Gonzalez]

Hi all!
I'm struggling for 2 whole days with OpenEJB embedded in Tomcat, and I'm
missing something VERY basic:
How can I setup a DataSource for my application?
I've seen the <Resource> in many pages, but it's for the openejb.conf
file, and it does not exist on the embedded OpenEJB.
I've tried to write a service-jar.xml in my classpath, but (not
surprisingly) it's not magically read.
Please, I need help in this one...