As you head into KCA, coming down from the bridge, take a left from the first roundabout. Then take your first right. The large blue and silver-gray building on the right houses the post office. There isn't a proper driveway, just follow the dirt track into the parking lot.
If you look at the building from the main road, what you see is a green sign for the "NBAD (coming soon)." The post office sign is around back, above the entrance to the post office.
its on the main road when you come in, you cant miss it, as its the biggest building there. its the big silver building but the entrance is off the little road, the backside of the building. it does have emirates post written over the top.