When I go to the indigo plateau it says lorelei is not there. What so ..

Go to One Island and retrieve the Ruby in Mt. Ember. Then, go to Four Island, and help Lorelei defeat the rockets. Go to Six Island and let the Scientist steal the Sapphire, then retrieve the Sapphire in Five Island in the Rocket Hideout.

After that, the E4 will be open again with much stronger pokemon.

You have to go and do the island quests. Lorelei should be on island 4.

She is on Four Island.

Lorelei is in the cave on 4 Island. You will need Waterfall to get to her. To get to Four Island though, you need to bring back the Ruby to Celeio. Go to Mt. Ember and battle the rocket grunts you saw there last time. When they leave, go in the cave (obviously ignoring there orders not to). The cave is pretty straight forward, but bring some repels if you don't want to run into wild pokemon. When you find the ruby, bring it to Celeio. He will give you a Rainbow pass giving you access to the Sevi Islands. You can go to 4 island now. To find the Sapphire he asks for, visit 6 and 7 island first and enter the password into the Team Rocket Warehouse on 5 island. To get the second password, go to 6 island and travel all the way south until you find some ruins and use Cut on the door. You can decode your way through the cave there but it will just say up, down, left, or right. Its braile, so you might be able to find the letters in a dictionary. I just counted to braile letters and guessed what it spelled. It wont't take very long. A Team Rocket Scientist will steal the sapphire when you find it but he will tell you the second warehouse password. Go to 5 island and find your way through the warehouse. Battle the Scientist and return the Sapphire to Celeio. Voalia! You may now Trade with the Hoenn region as well.

Have you already beat the Pokemon league 1 time. After you beat the Elite Four once Lorelie goes back to her home on Four Island.