To get into this exculsive club, you must have access to the Resort Area. Next thing, you must have 20-30 ribbons in total on all your Pokemon. After this, walk inside the building and the girl will then be about to take you out of the building when she sees how many ribbons your Pokemon have. Now, you have access to the whole building, including extra expensive ribbons for your Pokemon and a Relaxing Room for your Pokemon.
You must obtain the National Pokedex. (And it's PAL Park, not Pearl Park.)
Trade.
there are 150 kinds of Pokemon that you can see and/or obtain. Unless you have friend trade you Pokemon that you can only get in pearl to you, you'll never obtain them all. In Pokemon diamond and pearl all you have to do is see the Pokemon and you get its data unlike the older versions where you have to obtain them.
Migrate it.
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darkrai and arcus
a REAL diamond? lol in a cave, if the Pokemon one, download it or buy it
The only way to obtain Articuno in Pokemon Diamond is to trade it from GBA via Pal Park.
No, you do not need diamond and pearl to obtain all the Pokemon in the national pokedex. You either need to have a friend who has all the older Pokemon, or you could transfer the Pokemon yourself from older games.
In Diamond, you can obtain a Water Stone in the underground in Sunyshore City.
You will need to migrate from Pokemon Ruby/Sapphire/Emerald.
You must MIGRATE!!!!!!!!!!!!!!!!!!!!!!