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)
In Pokémon Silver, you can obtain the radio from the Goldenrod Department Store. Head to the top floor of the store and purchase the Radio Card from the lady in the corner. Once you have the Radio Card, you can use the radio function on your Pokégear to listen to various stations and access special features in the game.
talk to guy in radio tower
It works autimaticaly.
you use it on yourpokegear (the phone thing)
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