More details would be nice.
You refer to RBR but Skin Manager is more associated with RSRBR, although I know very well that you can use the same utility with some other installations as well (like Czech or vanilla).
What is the skin you're trying to add (if "adding" is indeed something you'd like to do instead of just replacing the existing one)? A very small minority of released skins are ready to be used with Skin Manager as such. The skins hosted by Rallyesim itself should be packed for this purpose but with any other releases you'd have to pack the .dds files to a separate .zip package yourself. And this .zip file should contain only those .dds files, nothing else, no folders or subfolders let alone anything else.
Basically, the zip package should go to \Skins\"Car subfolder with the same name as in the actual Cars directory"\"Skin title"\. Whether the "Skin title" and the name of your zip package should be identical or not, I'm not sure. If a skin release happens to have a preview pic, you can place it to the same folder with the zip file. (Again, the screenshot may have to be named exactly like the zip file, or not.)
