There should have been an option when the ticket was purchased
If you have purchased a general admission ticket you can also add into the basket with this a car pass for your time here. However, if you have purchased an admission ticket with either the Swindon Shuttle Bus or Cheltenham Park & Ride you will not be able to park at the showground. For these tickets, you will need to park either in Swindon or at Cheltenham racecourse. These bus tickets are only valid for the pedestrian entrance which is not accessible from the car parks.
I would email the customer service and they'll add them to your account. You can also go through the buying process then removed the day ticket so just checked out the car ticket. Bit of a farce, that's what I had to do with my bus ticket as it wasn't included with the FRIAT pass