At the entrance of the post office on the ground floor, face the street and turn left. Start walking and pass a multi-storey car park. The Hong Kong City Hall is the building next to the carpark. They are not exactly close to each other and there are some streets and roads in between, just keep walking, you won't miss it! It's a building with a garden and most of the structure has a rectangular shape which looks quite like a box, just like the Post Office.