The air cell should be at the larger, rounder end of the egg. Did you see the air cell at all when you candled it before now? There is a clear area at the pointy end of the egg as well while they are developing, it's just an empty area (filled with fluid, unlike the air cell) until very late in incubation when the duckling takes up all the space. It can kind of look like another air cell sometimes. Hopefully that's all it is because if the air cell really is at the wrong end then that often makes hatching more difficult (sometimes impossible) for the duckling.
Have you heard any peeping or tapping from the duckling inside yet?
Edit: I just went back and looked at your pics earlier and it appears the air cell was on the right end of the egg then, so I wouldn't be too worried. It's probably just the little bit of empty space left inside that the duckling hasn't taken up. The rest of the egg is dark now, right?