Go to the Radio Tower in Goldenrod City, where you can answer a series of questions by an NPC to get it.
In the radio tower after you answer a quiz
The EXPN card is on the first floor of the radio tower in Goldenrod city.
You need a special card to connect your radio to kanto's radio frequencies.
In Lavender Town, where the radio tower is, there is in old man inside and he will give you the Kanto Radio card (if it is your first visit)
talk to guy in radio tower
you use it on yourpokegear (the phone thing)
It works autimaticaly.
Lavender Town> Radio Tower> Someone
go the radio tower in goldenrod and take a quiz at the counter and she will give you a radio card
use the stylust and go to the top star on the radio.
You win the radio card for free at Goldenrod when you answer the questions correctly at the front desk
get a radio card and play the poke flut song on you radio