Almost all add-ons are packaged in a way as to present the directory structure and ssc files necessary for Celestia to parse and include them in the current execution. Almost all of THOSE are in .ZIP files or some intriguing variant like .RAR, whose unzipping/unrarring utilities will place them wherever you like.
To wit:
-A top level addon folder named <addon-name> containing:
-A "models" folder with the meshes used by the add-on;
-A "textures" folder with the texture maps that the models of the add-on use (usually contains a "medres" folder for the most common resolution used - high resolution or "hires" folders are usually available on demand) and
-One or more .ssc files to define in Celestia's universe where those textured models appear.
This toplevel directory and all of it's above listed contents gets placed in the "extras" folder prior to execution.
Hope that helps.
Also hope it's right - seems to make sense to me at the moment...