The Air Drop location icons look identical to the achievement icon - a box with a parachute. Some of these locations you will come across during your exploration (and you'll need to explore most of the map as part of the DLC, but the map itself isn't that big), however some Air Drops will only become visible once you clear Antenna areas.
There are 15 airdrops: 10 can be found through exploration; 4 of them are found through clearing the Antenna's missions; the final one drops out of the sky at the completion of those missions.
For those of you voting negative: hopefully this guide is now more useful. Let me know if I can make it better.