First, you have to beat Skyla the 6th Gym Leader, then you have to go into the house. (there is only one house in the city)
Talk to the people in the house and they will tell you that their 'treasure' is at the end of the runway. Their treasure is the TM Aerial Ace. Go to the bottom of the runway and at the left side you will find it.
Source for this is here
Hope this helped!