Talk to Director after you solved the Power Plant Mystery then go to Lavender talk to him you will get it
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get the EXPN card in lavender town to get that station
You go to your pokegear to use it. (radio) It installs itself.
You go on the Pokegear and go all the way to the top of the radio and that's the EXPN card (EXPN card is the poke flute)
The expn card automatically attaches to your radio now you can tune into kanto radio!
Go to the radio tower in lavender town then talk to the man in the hat and he will give you the expn card