I have the National Dex, Got the island one thing operating, and beaten the elite 4... What do I have to do... I also defeated the elite four...

Are you having problems with something specific? There's not a ton else in the game to do, if you've beaten the Sevii Islands quest, and the Elite Four, other than leveling up your pokemon and having fun.

I did everything... Did all the quest... Gave the Ruby and the Saphirre to Celio... and beaten the elite 4 and I still cant do anything about it! I just wanna catch mewtwo =( I need it for platnium

Well, it's been quite some time since I played Fire Red, but I vaguely remember having to beat the Elite 4 twice... once before the islands and once after... you may want to try that again. Don't take my word for it though...

I will at least put stock in it, because I remember hearing something like that as well... If not, I would be patient and follow the walkthrough, as pokemon_god suggested.

However, I will go out on a limb and say that your version is glitch-free...

AEPMT

To enter the Cerulean Cave (home of Mewtwo):
1. Beat the pokemon league (you only need to once)

2. Get 60 Pokemon in the Pokedex

3. Go back to Mt. Ember (1 Island)

4. Battle 2 rockets, make sure you have Strength, and go in the cave, and find the Ruby. (1st password obtained)

5. Take the Ruby to Celio, who gives you the pass for Islands 1-7 (Rainbow Pass)

6. Go to 4 island FIRST, enter the cave, find and teach waterfall. Beat Team Rocket in the cave.

7. Go to 6 island, go south with a Cut pokemon. Open the door, enter the cave, and fall through the holes in this order: UP, LEFT, RIGHT, DOWN. Try to get the Sapphire and a scientist steals it. (Both Passwords obtained)

8. Go to 5 Island. Head East (band reference!) then south until you find the Rocket Warehouse.

9. Find your way through the puzzle (and decently strong trainers) but don't forget to find the items along the way including a Sludge Bomb TM. Beat all of the Rockets, they finally disband, and then beat the Scientist. You finally get the sapphire.

10. Give Celio the Sapphire to get full 3rd gen. trading abilities as well as access to Cerulean Cave!

I HIGHLY RECOMMEND not to use the Master ball on Mewtwo unless you don't want the Legendary dog.