This place is called "Hobak" (pumpkin) massage.
It's on the same street as Black Hole Anma and is near Garasugill (of you're familiar with the place).
Go to Sinsa Station Exit 8 and walk straight out. You're in for a long walk though (about 10 minutes).
You're going to pass Black Hole, Garasugill and a KB bank. Just keep going until you reach the Andre Kim store on your left. You're close, just a bit further.
Walk past the gas station and keep an eye out on your left. You'll see on the second floor, a long horizontal yellow sign with a picture of a pumpkin on it. On the ground floor will be a Maria Louise store. This is the building.
To get it, you have to make a left into a narrow alley way. If you hit the huge Hi-Mart electronics store, you went too far.
On the side of the is building the entrance. Yeah, discreet.
If you can speak Korean or feel comfortable enough asking the hotel concierge, call in to make an appointment.
02-3446-4635 It's so much easier and no chance of a disappointment if you make a reservation. I highly recommend you do so.