You would have to purchase the corresponding DLC.
I believe you are referring to this one: Need for Speed™ Hot Pursuit Porsche Unleashed Pack
The Porsche Unleashed Pack includes three very special Porsches, each one created to fulfill a very specific purpose - the power of the 1982 Porsche 911 Turbo, the speed of the 1988 Porsche 959, and the style of the 2011 Porsche 911 Speedster. It also includes ten new events to drive them in that will test your Racer and Cop abilities to the limit.
