Make sure that you have downloaded and installed your DLC packs/cars from the Microsoft Store on either PC or Xbox.
When you are in the game, you can either travel to the Horizon Festival or to any House you own to access the Autoshow.
Navigate to the DLC Cars Section
Select View all cars
From here you can 'purchase' your cars for FREE. Once they are purchased they will appear in your garage.
Only the account that purchased the content will be able to buy the cars for free. If you have done all the above steps and are still being asked to use in game Credits to purchase your DLC cars, please submit an DLC Issues Ticket.
If you are trying to select a car and you are getting a "This content is already purchased" message, then the car is locked behind some kind of task. Example, the Lamborghini Aventador J, is unlocked by completing the last clue in Fortune Island.