The house door faces the square of the Persian Trade House, to the west; it also has a back door to the canal facing the Cathedral. (Click map to enlarge.) Scarlett may have to pick the lock.
Scarlett should return to the Outer City, and go into the Trading Center area. The medicine is on the highest rooftop catwalk. Any route to the top will do, but the most straightforward is to enter the Garrison and take the stairs to the top, pass through the jail area and out the door. (Click map to enlarge.)Guardsman standing watch over a chest which contains the medicine. The guard is corrupt and can either be bribed or killed, or select find peaceful solution twice and guard will give you the key to the chest.
After collecting the medicine, Scarlett should return to the citizen's home to collect her reward and complete this quest.