The AirportVehicles extension requires a separate activation with a one-time internet connection. Once the purchasing process of your favorite online store has been completed you will receive a serial. 

To install the extension open the Content Manager Application for your corresponding OS which can be found in X-Plane\Resources\plugins\SAM and download the extension. 

To do so you will be requested to paste your purchase email adress and the received serial.

If you have any problems please open a ticket.