you need the carveiwer files installed which can be found on this page here: BullRing Motorsports
If you already have the files but can't get it working please be more specific on what exactly is not working so someone may be able to assist you further on the issue
Save your flats as card.bmp in the folder where your gen6 x files are located. Open car viewer, click File, then Open DirectX mesh. Choose the make you're working on and you should be all set.
1) Download (You already did)
2) Remember your location you saved the files.
3) Go to save your flat (export, save as, etc.) menu.
4) Find the location of the CarViewer files through that menu.
5) Go to the folder of the make you want. Click on it.
6) Overwrite card.bmp.
7) Open CarViewer.
8) Go to File>Open Direct X Mesh.
9) Find the BRGen6 Carviewer files again.
10) There will be car.x and car_wGlass.x options. Open whichever one you want.
11) After loading, Voila! You have successfully opened a BRGen6 flat in CarViewer.