2. You also need to get the "suite key" item via mystery event. Basically same process.
Has that got something to do with the girl in the village area just after one of the lakes? she's standing outside saying she's lost her key etc
Actually, that quest is unrelated. What you need to do is help out the Sailor's Son by getting the Lunar Wing. Note you have to have the national Dex to do this event and the Darkrai one aswell. Here's the step-by-step way to get Darkrai (and in the process, meet Cressalia):
1. See all 150 pokemon in the Sinnoh dex, then go to the professors house (the place where you originally got the pokedex) and you'll recieve the national Dex from Prof. Oak.
2. Go to Canalave and enter the house right next to the sailor and talk with the boy. After the event, go outside and talk with the sailor and agree to help. You'll be sent to Half-moon island where you'll meet Cressalia. After it flies off, get the item it dropped and return to Canalave.
3. Go back to the house with the boy and talk to him to use the Lunar Wing you got earlier. He should nowbe healed.
4. Go to the abandoned Inn in Canalave and it should now be open. Enter and watch the cutscene and you'll then appear on full-moon island where you will meet Darkrai.
5. Defeat and/or capture Darkrai.
6. You will wake up back in Canalave and you now have the Darkrai pokemon.
7. ???
8. Profit!