The GroundService 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.