How do you get the suite key on Pokemon diamond?

Answer 1

Go to the main desk in that area (not inside, just outside) then use the Dowsing machine.

Then stand in front of the hidden item and press A.

The girl will thank you, go inside and give you a Lava Cookie

Answer 2

I suggest using the Dowsing App that Dawn/Lucas gave you and slowly searching, if I remember right it's not too far from the South exit of the Resort.