Please someone tell me how to get meteor cave.?

After you beat the game, go back and recruit Articuno, Zapdos, and Moltres. Then speak with Alakazam. He will tell you to visit them at their Friend Area: Legendary Island. When you talk to them you will be pointed to Silver Trench where 99 floors and a Lugia at the end. When you recruit Lugia, the next day Xatu will be standing outside of you base. You will then be able to go to Meteor Cave where Deoxys lives.

Recruit Lugia!!!!!!!!!!!!
Beware.......!!!!
In every floor there is a mirage deoxy Attack,Defense,Speed,Normal Mirage deoxys. Be Careful!!!!!!!

First you must recruit all 3 legendary birds-go back and rebattle them until they join. Then talk to Alakazam, he tells you to go visit them in their friend area(Legendary Island)...things will happen which will lead to the opening of Silver Trench-you'll need the HM Dive (or a poke who knows it) to get in. It's a tough 99 floors until you meet Lugia. Defeat him and recruit him. The next morning Xatu will visit you...Meteor Cave will show up on your list. Warning! Only 1 Pokemon allowed inside, and only 3 items in inventory, and there's only $ inside!
That and Deoxys, of course!! (Luckily it's only 20 floors) To get to another floor you need to defeat one of each Deoxys form: Attack, Defense, & Speed-Normal is only on floor 20. To recruit him, defeat him at a high enough level and team ranking, and a friend bow can't hurt either.
That's the story-good luck!

You have to recruite the three legendary birds then go to recruit Lugia then talk to Alakazam then you go to silver trench and then talk to xutu and then you will beable to go to meteor cave were deoxys lives.

You HAVE to recruit Lugia in Silver Trench! He's lv. 30, and has gust, and is at floor 99! You unlock silver trench by recruiting ALL 3 legendary birds!