After you beat the 6th gym leader, talk to some kids in a house beside the "airport"...they will say they put a tm on the runway.
It's the same thing in both games. if you go in one of the houses in mistralion city, a boy will tell you he lost a tm. then go to the end of the runway and the tm should be there. when you find it, they let you keep it
TM40 Aerial Ace is only accessible after you defeat Skyla at Mistralton City's Gym. After you do that, just go to the very bottom-left corner of the airport runway and you'll find the TM on the ground. This tutorial has screenshots that show you exactly where to go. And yes it should be in the same location for both Pokémon White and Pokémon Black.
