I tested to see If I could open Automobilista car dds files, and mine opens with no problem.
So just a couple of silly questions.
Did you put the dds plugin into the plugins folder in Gimp?
Which dds file are you trying to open? Have you got the correct one?
Typically I open the cars skin file, which is located in the cars team folder. You can see which one it is by reading the VEH file. The skin is listed on the first line of the file.
eg: DefaultLivery="Caterham_270_01.dds"
The only other thing I can think of is maybe you have the wrong plugin. I got mine from the Race Department skinning guide, available here. It contains a tutorial and the plugin files.