PokéBase - Pokémon Q&A
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Best answer

You need the game My Pokemon Ranch.

>When 999 Pokémon are present, the ranch will expand and jump to level 25, the maximum level. Once this level is obtained, a new wanted Pokémon will be added. Hayley is now asking for any Pokémon Egg. When the egg is brought, she will offer to trade it for her Mew. -Bulbepedia

Once you get the Mew, trade it to your Diamond game.

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wht do u mean when all 999 pokemon r present?
You need 999 Pokemon there.
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Its an event, which has already passed. sorry bud