Simply save your car as a .jpg, choose one of the four radio buttons as the slot you want to work with, upload your texture, then click on the box that corresponds with the radio button to see your texture applied to the model. Only a Chevy is available to work with at the moment.

Keep in mind that the model they use come from the beta demo that everybody has now. Your current texture will probably need to be readjusted to fit the finished model. Also the watermark that obscures some of the viewed car is because the viewer they are using is unlicensed, so don't complain that White Falcon didn't want to shell out $500 to license it.

This viewer is just to give you a sense of how your Chevy will look in N2003.