How to Get Darkrai in Platinum Without Event
If you really want a Darkrai and missed the Nintendo Event that gives Member's Cards for the inn in Canalave City, here is how you can get the Dark Pokemon without having to trade. Of course, this can damage your game, but the chances of any problems are very low. First, get your Action Replay and enter the following code.
Membership Ticket (to get Darkrai)
94000130 FCFF0000
621BFAF0 00000000
B21BFAF0 00000000
B0000004 00000000
0000b600 EDB88320
Select it as one of the special features of the game you included and start your game.
If you are not there, go to Canalave City, the port city across a small lake (not a main lake like Verity, Acuty, Varlor and Spring Path.) Once you've been to Canalave city, go to the Pokemart and get the Member's Card. When you enter, the green-suited delivery-man next to the counter will give you the Member's Card only if you press the L+R buttons as you enter. Once you receive the card, go north, and go to the house closest to the bridge. Now, you will discover you can enter. Member's Card permits entry to the Inn. Once you arrive, a man will tell you to go to sleep. You will wake up on New Moon Island. You will see the legendary Pokemon, and you have to battle it. I'm not sure if you can battle Darkrai again. If you can, catch it. To be safe, save first. That way, you can battle it again if it faints. That's how you get Darkrai.
To get Arceus you need to go to all the Pokemon Centers in Sinnoh and once you've been to the one in Sunnyshore City, talk to a lady in the union room and she will give you the Azure Flute but you have to have Wi-Fi, there are many more ways to get Arceus. It might not work. Try the Survival Area, Battle Frontier and the League's Pokemon Centers.
I don't know how to get Shaymin, but youtube.com has the answers. There is a video there that shows how to glitch your game, and it is done on a Pearl. You get Shaymin that way, although it is likely to hurt your game. I think it doesn't work on Platinum, and the only way to get the Member's Card is to receive it as a Mystery Gift or to hack glitches in the game. (Only during Nintendo Events, and the Nintendo Events only happen in Japan now. It seems like Nintendo will release Events towards the end of 2010 and the beginning of 2011.) Glitches are risky and could permanently or severely damage or even delete your game. Of course, it is still an option, although risky, and it is the most popular way to get Pokemon. I don't recommend it, and people who are desperate can cheat, but the only way to keep your game safe is to wait for Nintendo Events, or someone who is willing to trade you. I used an Action Replay, and my game is fine.
Now you can also get a shiny Pichu, Suicune, Entei, and Celebi. Go to mystery gift to get them all. Youtube can give you videos on how to do it.
How to Get Darkrai in Platinum Without Event
Source: https://www.fanpop.com/clubs/pokemon/answers/show/14533/help-how-darkrai-shaymin-arceus-without-action-relpay-help