Well, make sure you've beaten the Elite Four and grabbed the National Dex from Prof. Oak. Then go to One Island and talk to Celio. He'll ask you to help him fix the Network machine. Go to Mt. Ember to find the Ruby, then bring it back to Celio and he'll give you the Rainbow Pass which will allow you to go to the other Islands.
There's a good walkthrough here if you get stuck: http://pokedream.com/games/fireleaf/...-island1_2.php