1. Turn on your Xbox 360 or PlayStation 3 and connect to the internet.
2. From the main menu, navigate to the Xbox Games Store or PlayStation Store.
3. In the search bar, type "Red Dead Redemption Undead Nightmare."
4. Select the game from the search results and click on "Buy."
5. After completing the payment, the game will start downloading.
Steam (PC):
1. Launch the Steam client on your computer and log in to your account.
2. Click on the "Store" tab and type "Red Dead Redemption Undead Nightmare" in the search bar.
3. Select the game from the search results and click on "Add to Cart."
4. Proceed to the checkout and complete the payment.
5. After the payment is complete, the game will be added to your Steam library. You can download and install it from there.