Fix the power plant then go to the radio tower in golden rod then there should be a man that wheres brown and talk to him I did this a long time ago he might ask for something I don't remember
How Do You Get EXPN Card In Pokemon Soul Silver?
You can get it from the radio tower after you have fixed the problem at the electrical plant. :)
After defeating the elite four you have to go to the radio tower in kanto and talk to a person in there and they will give it to you.
Talk to the guy in the hat in the radio tower