You'll need the membership card to get in.
Here's the Action Replay Code for it:
94000130 fcff0000
b21c4d28 00000000
b0000004 00000000
0000aa3c edb88320
2000aa68 0000000a
d2000000 00000000
Go into the Pokemart and Press L+R then talk to the man in green.
When you enter the building you should start up the Darkrai event.
He's at level 40 in diamond and pearl.
The Harbor Inn in Pokemon Diamond is only accessible by using the Member Card. This item is only obtainable through an official Nintendo event.
practice on spelling for darkai diamond
At the harbor inn at canalave city. You need a member pass which you can only get through a Nintendo event
you look for it
In harbor inn
Action replay
you have to cheat
You could have only gotten a Mystery Gift through Nintendo in order to access the Harbor Inn in Canalave City in Pokémon Diamond however the key that's needed in order to gain access to the Harbor Inn, the Member's Card, it was never released for Pokémon Diamond so the only way for you to get the Member's Card is to use a cheating device such as an Action Replay or Pokésav.
In Canalave City above the Pokemon center
In Pokemon Diamond, Pearl and Platinum Darkrai can be found on Newmoon Island. To get to Newmoon Island you need the Member Card for the Harbor Inn at Canalave City.
you need a mystery gift
You must get the Member's Card via Nintendo Event to get into the Inn in Canalave City.