How do you get secret key in Pokemon HeartGold?

Answer 1

In Heartgold and Soulsilver, you don't need it. Put Rotom in the first slot and enter Silph Company in Saffron City.

Rotom will allow the elevator to work, and you will notice 5 items in the room:

Lawn Mower

Refrigerator

Fan

I think washing Machine

and

I think microwave oven

go to the item that you want Rotom to turn into.

and don't forget to read those notes in the room.

You can have all 5 forms of Rotom

6, if you count the original form.