First of all you have to subscribe to JabRSS's presence (I am running a JabRSS server with JID jabrss@cmeerw.net) and have to accept the subscription request from JabRSS. Then you can start using it by subscribing to your favorite RSS headlines by sending a subscription command to JabRSS ("subscribe http://some.url/path/to/rss", e.g. send "subscribe http://slashdot.org/slashdot.rdf" to subscribe to Slashdot headlines; or better yet try "subscribe http://cmeerw.org/blog.rdf" which is my Weblog where I will announce JabRSS updates).
If you are looking for other RSS sources, you might want to take a look at Syndic8.com - just search for a feed and subscribe to the RSS URL via jabrss.
There is also a short description on using JabRSS in Spanish on the Biso*os Usuarios de Linux de Mallorca y Alrededores web site.