You simply have to defeat Marowak's ghost at Pokémon Tower in Lavender Town with the Silph Scope you get from Giovanni (Rocket Boss) in the basement of Game Corner in Celadon City. You then defeat the rockets upstairs and talk to Mr. Fuji who will give you Pokémon Flute.
Then, after you have done this, you should be able to get passed the guy blocking the lift.
Hope I helped!
