Hi,
I've noticed that many addons use uppercase directory names, like
Textures\Hires\texture.jpg
Addons using this will not work on Unix/Linux systems, because on Unix-like systems "textures" and "Textures" are two different directories. So please make sure case is correct, i.e. all lowercase for Celestia directories (textures\medres, models, etc.). If you are referencing a texture or other data from a .ssc/.dsc/.stc. file, please make sure case matches what you are actually using, i.e. don't use Texture "mytexture.jpg" when the file is actually MyTexture.jpg.
Thank you,
Harald