^^^You don't. At the very least, if a car is part of a car pack but is available to purchase separately (some cars are available as part of a pack only), you'd have to purchase that car from the XBL marketplace...which adds it to the appropriate dealership
in-game. You still need in-game credits to purchase it for career play. At this point, you can either purchase it with credits, tokens, or have someone gift it to you if they're feeling generous.
As I mentioned above, you can't do that. If a car is part of a DLC pack and is not available for sale on the marketplace separate from the rest of the pack (some are, some aren't, depends on the car), then buying the entire pack is your only option.