How to get an Uber from HKG as of April 2017
Tl;Dr: Go to 2nd floor of car park 4, head to its SE corner, and the driver will pick you up in an outdoor area (no roof).
After twenty minutes of frustration due to Uber's insufficient outline/directions/documentation here:
https://www.uber.com/airports/hkg/
or in the app, and a lack of any informed airport employees regarding Uber (evidently not too popular of an airport option), here's good directions, at least for one pick up option:
After selecting a car type, you will be prompted to choose a pick-up in car park 1 or car park 4 (airport hotel car park). If you choose car park 4, follow signs. If you are around the southeast corner of the car park 4 on the car park's level 2 (the level of the car park probably is not consistent with the terminal level), there should be an outdoor area, where you can see the airport hotel building. The Uber driver will pick you up there.