We had a similar problem in our multiple flats building. Our bathrooms are aligned with the above unit that was leaking. But anyways, you need to open up the ceiling where it is leaking down from and hopefully it is right there above as opposed to water traveling across the room since maybe not level. If you are not much of a plumber then have a pro do it either after you open it up or have him do it all. I suppose you can do the opening and drywalling and painting if you want to save some money.