You have to have an action replay to get to Darkrai.
If you get Darkrai via a event on platinum it will be at lv.50
No.
Ok the answer is yes, but only throught the darkrai event
you can not catch Darkrai in platinum you can only get it at special events or you can go to GTS and trade someone for it the last option is to get from a friend or use a master ball on a wild Darkrai or on someones caught Darkrai.
In taco world
You do the same with the member key
new moon island
action replay
Darkrai
The only way you can get Darkrai is to hack the game or get it from someone who went to the event.
you can find creselia on moonlight island but i dont know about darkrai.