Use your itemfinder thingie on your poketch near outside the front desk.
you look for it
you have to cheat
you have to get a Pokemon to level 100 then talk to porfessor oakit has to be a special event
the answer to this question is to get out of your mums basement, get a job and stop posting questions about Pokemon :D
you you your ap dowser
To get inside the Harbor Inn on Pokemon Platinum you need the Member Card key event item. It is currently not possible to obtain the Member Card as the distribution events are no longer being held. The only way to get the Member Card is via a cheat device. The Harbor Inn has a NPC that can take you to the Island that houses the Pokemon Darkrai.
In harbor inn
Action replay
In Canalave City above the Pokemon center
The Harbor Inn in Pokemon Diamond is only accessible by using the Member Card. This item is only obtainable through an official Nintendo event.
you need a mystery gift
You can only get in the Inn with the Member's Card which is a key item that is an event item. Only Nintendo can give out the event items. They're not available anywhere in-game. unless you cheat with an ar