Try this:
1. Go to your WindowBlinds folder.
2. Delete the skin folder for the skin that won't install.
3. Delete the .cat file(s) from your WindowBlinds folder.
4. Reinstall the skin.
5. Open Display Properties and see if it is in the skin list now.
That usually does the trick. The .cat file (there is one for each user account on your machine) keeps a list of what skins are installed. Sometimes, if things go haywire, it stops keeping track and you don't see newly installed ones in Display Properties. Deleting it forces WB to rebuild your skin catalog, which usually fixes a problem like this.
Good luck. 
