The exact address I am not sure about, it is one of the small side streets starting from "calle Assaonadors", very near "Calle Princesa". Just ask any "night-type" person you see walking around in that neighbourhood, and he or she will know. The club can only be recognized from the outside by its grey door on which you have to knock and then wait until they let you in.
Over the years, the bar has changed a lot, and it has been at least 4 years since I have been there. At first, it was a very excentric place, with old sofas, littered floors, pool tables and a lot of artists, grungy types and alternative people around. Later, it got more "nightclubby". But it always remained sleazy, with a lot of drunk people who would go on until late in the morning (I have never actually seen the bar close). It was a wild, sleazy place, and weird things would happen sometimes.
The next link will give you a very accurate impression: