Due to a possible bug within X-Plane 12 itself which Laminar Research are investigating, there is a small chance that pressing the APPLY button after creating a Dynamic Livery may not apply the livery to the exterior of the aircraft. 

If this occurs, press the APPLY button again and the process of applying the Dynamic Livery to the exterior of the aircraft will start over. Alternatively, select Developer > Reload Current Aircraft And Art from the top menu bar and this should force the Dynamic Livery to apply.

Note: We would always recommend that you use the latest version of X-Plane 12. This will ensure that any fixes created by Laminar Research are available to you in your simulator.