On the paper you've found in Kurtz' stomach, you can see an address of opium den, Berner Street 13. You should go there - it's a first cross street behind the cemetery, while going down the main road.
March forward and turn right on a crossroad. Den is at the end of the alley. Go inside.
After talking to receptionist, go deeper inside, behind a red curtain. You get to the first den's room. Go to the second one (door on the right). From this room go forward.
Notice a bodyguard standing on the right, who is guarding the closed door. Try talk to him, but he isn't friendly. You can't do anything with him.
Go to the end of corridor and look at the last door on right, with a chinese writing over it (remember it). It's, of course, closed (check it).
Holmes and Watson reckons that you can get to the guarded room via the one with chinese marks, but you need to get a key to open it.
Return to the reception and look at keys hanging next to porter's desk. Holmes will find the one with identical chinese signs. You just have to lure out a Chinese who's welcoming new guests.
Return to the smoker room and pick up a glass of dirty water from a table on the left.
Go further inside and pick up ashtray with ashes from a table on the right (by a half-lying man).
And from a table on the opposite site you have to pick up a cup of Jasmine tea.
Open equipment. Move glass over cup of tea, pour it in and add ashtray. You get a lousy booze. Hand it to the lying man in the first room. After drinking it, he'll feel very badly what will lure a porter.
Use a moment and take key from reception.
Go to the end of den and open closed door with chinese signs.
Inside, below the curtain, on the left, notice something like thin door without door knob but with a grilled window. Hear the discussion of two men - they turn out to be Bishop's murderers.