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Get a rotom
Use the Rotom you caught in diamond, pearl and platinum.
bring rotom and make sure he is first and go in the elevator and you will see rotom forms
Get a Rotom and go in the elevator and Boom B1
In HeartGold and SoulSilver, you can access the 1st floor of Silph Co. as soon as you enter Saffron City however you can't go into the elevator in order to get to the 2nd floor unless you have a Rotom as your lead Pokemon.