I bought the Rockstar car pack and it says the vehicles are installed but I cant access them in the garage.
If you mean that the cars are not in your garage, installed DLC does not automatically put them there; you’ll need to ‘buy’ the cars from the Autoshow or Buy Cars menu to transfer them to your collection.
If you mean that you’ve attempted to transfer the free car of each model and the game instructs you to purchase the DLC that you already own, you might try rebooting the console. If that doesn’t work you may need to contact forzafb@microsoft.com or www.twitter.com/forzamotorsport or /xboxsupport for help.