In order to activate the product please follow instruction below:
Option 1, from a web browser:

Enter https://play.google.com
In the left menu, click "Redeem" (a message that open the session with your Google Account, will appear if you have not already).
Enter the promotion code in the appropriate field and click "Redeem".
The game associated with the specified code will be registered in your library and you can install Android on the tablet SHIELD.

Option 2 app from Google Play tablet SHIELD:

Open the Google Play Store application of SHIELD Tablet.
On the home page, click the icon in the upper left corner to open the menu.
Select "Redeem".
Enter the promotion code in the appropriate field and select "Redeem".
The application will direct you to the download page Game Play Store, where you can install.