what expn card<-----Why in the world did this person answer this and not know the answer?
Anyway you just get it to be able to use the radio on the pokegear in Kanto.
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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 lavender town and go in the radio station. Then there is a man facing the wall so talk to him and he will give you the EXPN card.
You go to your pokegear to use it. (radio) It installs itself.
go to lavender town after u beat the elite 4, and then go into the radio tower and talk to the gentleman wearing the brown hat by the PC