To catch Darkrai in Pokémon Platinum, you need the Membership Card. Unfortunately, the only way to get this item was to participate in a Nintendo event that ended in 2009. You can still have a Darkrai, but you need an Action Replay to modify the game. In this article, you will find instructions on how to use Action Replay to receive Darkrai in Pokémon Platinum.
Steps

Step 1. Obtain the National Pokédex from Professor Oak
You can do this after beating the Elite Four. You can find it in Professor Rowan's laboratory in Sabbiafine.

Step 2. Awaken the son of the Sailor Elfio
You will find it in Canalave City. To cure him of his nightmares, you must travel to Fullmoon Island and obtain a Moonwing from Cresselia.

Step 3. Save the game and turn off the console
You must use an Action Replay to get the Membership Card.

Step 4. Purchase an Action Replay
This device allows you to use cheats in many games. You can buy it on the internet or at a video game store. Make sure you buy a model compatible with your game system (Action Replay for DS, DS Lite, DSi, 3DS, etc.).

Step 5. Add the codes for the Darkrai event into your Action Replay
You can do this by connecting the device to the computer with a USB cable and entering the codes using the cheat manager program. Use only one code at a time and enter it exactly as indicated, respecting capital letters. The codes are as follows:.
- 94000130 FCFF0000
- 62101D40 00000000
- B2101D40 00000000
- 0000B5D4 00000008
- 0000B5D8 00000000
- D2000000 00000000

Step 6. Launch Pokémon Platinum with Action Replay
Put the game cartridge inside the device, which you will insert into the cartridge slot. The game should be facing you.

Step 7. Hold the R + L keys while entering a Pokémon Market

Step 8. Talk to the man dressed in green
This character, present in all Pokémon Market, will give you the Member Card.

Step 9. Go to the Sailor's Inn
This is the small house north of the Pokémon Center in Canalave City.

Step 10. Talk to the man
He will make you lie down on a bed and you will reach an island in a dream.

Step 11. Enter the forest
It is in the north part of the island you are on. Climb the stairs and follow the path until you see the opening in front of the forest trees.
Once you get to the island, it's a good idea to save your game

Step 12. Talk to Darkrai
You will see it floating on the lake. Talk to him to start the battle.

Step 13. Capture Darkrai
He is level 50. It won't be easy to catch him, so make sure you have plenty of Ultra balls, Dark balls (if you want to try to catch him at night) and Timer balls.