Here is how to unlock the downloadable cars for offline play. You need to unlock their equivalents in the offline game in order to use them.
To get the Spike TV Car you must unlock the Black Elite, to get Dolby car you must unlock the Etnies Racer and so on.
To unlock the 360 car get to Dominator Rank and win a medal on Lone Peak Crashbreaker Race Reverse.
To unlock the Circuit City Car get to Assassin rank and win a medal on Central Route Crashbreaker Eliminator Short Forward.
To unlock Best Buy car complete Motor City Challenge Sheet.
To unlock Gamestop car get to Assassin rank and win a medal on Central Route Short Reverse Traffic Attack.