Here's why:
* Darkrai is not obtainable in Pokémon Diamond. It is a legendary Pokémon exclusive to Pokémon Pearl.
* The boy you wake up is related to the Lake Guardians. Waking him up is a quest that involves finding the Lake Guardians: Uxie, Mesprit, and Azelf. This quest is important for the overall story and does not lead to a Darkrai encounter.
To obtain Darkrai, you need to play Pokémon Pearl. It can be found in the Newmoon Island area.
If you're looking for another legendary Pokémon in Diamond, you can find Shaymin in the post-game.