The Airfield is located in the centre of the map in Silver lake on
Nelson Way, there is one entrance which is on the right side of the road when travelling south of the map (Away from the Junkyard).
Here's a map with the entrance location
Once you are driving one of the toy cars (Id suggest the
Toy P12) head into the Airfield and use any of the split ramps to perform a barrel roll, you will need to get one half of your car on one side of the split ramp and the other on the smaller ramp to perform a full barrel roll. A run up and use of boost is suggested.
You will also unlock the following if you havnt already
Here's a video