Select Activities, then select Aerocache Hunt. This will bring up a list of all the Aerocaches that can currently be collected. The top one of the list will have a timer next to it; this is the Aerocache of the day. Find this on 50 different days and the achievement is yours. See "Aerocache Hound" for more information on finding Aerocaches.
Note: While this achievement may be possible without DLC, it will take a much longer time for them to come up. Therefore, it is recommended that you buy the Hawaiian Adventure pack to make this much quicker.