Dang I beat the Elite four and gary and still can't get in and catch Mewtwo..

You have to be able to trade to Ruby and Sapphire first.

You have to get the Ruby and the Sapphire first.

Skip to the bottom of this guide if you already got the Network Machine running, otherwise, read the whole thing.

Celio of One Island is asking for your help to gain two precious gems. They are known as the Ruby and Sapphire gems. The Ruby Gem is located on Island One. Go to Mt.Ember's cave. You can't enter it just yet because two Rocket Grunts are blocking your entrance. It's not hard to beat them, so beat them and enter the cave. You got the Ruby Gem, take it back to Celio and he'll ask you to get the Sapphire Gem. This is quite annoying to get, but if you really want to trade with Ruby, Sapphire or Emerald, you'll need to do this long and boring task. To start, you need to leave to get the Rainbow Pass. Get the Rainbow Pass, and go to Four Island. Make sure you have a Water type with you, because you're going to go into the Icefall Cave and get HM Waterfall. Teach it to somebody and go up the Waterfall. Lorelli from the Elite Four is there. She's being trapped by some Team Rocket guys. Beat them and Lorelli wants to save the other Pok�mon. Go to Six Island and go to the Pok�mon Center and get someone who knows Cut if you don't have someone who already knows it. Find your way to this cave that is sealed. Use the move Cut on the door and it will open. This area is known as the Dotted Hole. Enter Dotted Hole. The Sapphire Gem is here. To get to it, you need to reach the bottom by going down the holes. Go to the top one first, then the left, then the right and the bottom lastly. You will end up at the bottom, woot. Go get that gem! What the hell, a scientist takes it from you, yay. What's the upside to that? You get the last password to the Rocket Warehouse. Go to Five Island and go to Team Rocket's Warehouse. Now, enter it and beat everyone in your way. The puzzle is annoying to solve and is stupid, but once you reach the top, you find the Scientist and you need to battle him. He's not too hard because his Levels are around 47. Beat him and you'll have the Sapphire Gem. Woot. Back to One Island, give the Sapphire Gem to CElio. The Network Machine will be running, enabling you to trade with Emerald, Sapphire, Ruby, XD and Colosseum. Now, you can go back to the Elite Four and battle them again. They're higher in level now and some Pok�mon have evolved. This gets crazy, bring lots of items to recover with! When you beat them, you can go into Cerulean Cave and catch Mewtwo.

Go talk to celio and go to mnt.ember and fight the rockets and go in the hole. Then get the ruby take it to celio and he will give you a pass then go to dotted hole on 6 island use cut at the door and find the sappire .
A scientist willsteal it and give you the other password to the rocket warehouse then go and beat all the rockets in there and the scientist will battle you after you beat him he will give you the sappire then take it to celio and the guy in front of the cave will move