Head towards the river, turn around and find a metal gate #1. Approach the gate and wait until Vito opens this passageway. Head forward once you're in the sewers. Make a stop near a hole in the tunnel #2 and wait until the dog stops barking
Continue moving forward. It would be a good idea to explore the left tunnel along the way #1, because you'll find a Playboy magazine there. Keep going straight until you're given a chance to leave the sewers #2.
Press the crouch key (C) immediately after leaving the sewers and then quickly take cover behind the dumpster #1. One of the guards will show up here. Wait for him and the dog to leave and also wait for other guards to escort the prisoners into the slaughterhouse building. Find a new container behind you and climb on top of it #2.
Climb to a small roof and then to a pipe #1 and a metal balcony #2. Proceed towards an opened window. Thanks to this you've just gained access to the main building.
FINDING THE CAPTURED MEN:
Make sure that Vito is crouching (C key), because you can't be discovered for the next few minutes or the hostages are going to be executed. Ignore the sleeping guard and instead search the area near the dumpsters to find a Playboy magazine #1. Leave this room and carefully approach a new corridor #2. You'll watch a short cut-scene here, showing you where the captured men are being held. Saving them obviously is not going to be a simple task.
Slowly start moving forward. Take cover behind a small cover #1 as soon as you hear the sound of a sliding door. Wait for one of the slaughterhouse workers to exit the building and proceed towards the room he came from #2.
You'll have to be ready, because a second worker will show up here very soon. Take cover near the opposite exit from this room #1 and wait for him to show up. Slowly approach him from the back and silence him by pressing the left mouse button #2.
Remain in the same room and go to a closed door #1. You could also proceed to this door immediately after avoiding the first worker (the one who left the building), but it would be quite risky. Enter a new room, turn left and then turn right #2 to get to a much larger area.
Make sure that you're crouching and quickly take cover behind the counter #1. Notice that an armed opponent arrived here are there are also two slaughterhouse workers in this room. Start moving right #2 while staying close to cover.
You'll soon reach a corner and you should leave cover for a second or two to collect a Playboy magazine found nearby #1. Make sure to return to cover before the blue meter shown near the radar is full. Continue moving right (press the E key each time when you want to get past a corner) #2.