Hello Mark,
The TWR-K53N512 system features a MK53DN512CMD10 MCU, when you create a new project in keil you should use this MCU.
Also, it is supported by the Kinetis SDK, if you are interested in this SDK, I recommend you to take a look into the Kinetis Project Generator where you can create a new project for your MCU with the Kinetis SDK support in Keil.
Best regards,
Earl.