There is one around Hilton. It's near Nandaemun Market Station (Metro No. 424 or 423 Station) Exit 7. When you come out from exit 7, go straight and then turn left at the first corner. There's a small booth in front of a steak house (Outback or something I think). Go inside and give the cards to the guy and he knows what to do. Normally it 95% for 50,000 and 100,000 card and 96% for 300,000 and 500,000 card. You shall see Sinsegae Department store when you come out from exit 7, if you don't, you are either in the wrong station or wrong exit.