Hi m8
Best to post in the computer help thread.
Nascars will show if you havn't installed the updates from the Ukdirt site, the updates have the .car files (chassis/models)
Red skin will show if you havn't the correct tex file and or drivers txt details added.
If you want to race say a mk3 cortina, you would paint the skin, convert it to a tex file, this tex file then needs to be put into your 'under2lt' folder within the ukdirt bangers folder.
In the same folder you will find a txt file named 'drivers' you need to add your txt details to this, copy an entry from another driver and paste it into the drivers txt file, now edit it and add your details, chassis name, number & your name.
More details about this can be found at the link below, a basic guide but should help. Try and add a skin using the guide and you'll then have more of an idea when you want to add yours.
CLICK HERE