I thought I had been very careful in getting it into the right directory, but instead it unzipped as a folder called arrow mod (or some such) in the datafiles directory, with sub folders for meshes etc all of it's own.
In the end I just copied and pasted all the downloaded files to the correct folders. I copied anything in the arrow mod\meshes directory into the datafiles\meshes directory - if there was a sub-directory in the downloaded arrow mod\meshes directory, I created and copied to it in the datafiles\meshes directory). I kept on going like that till all the meshes and textures and sounds where in the right place, then I copied the executable file of the mod to the datafiles directory.
OK - a bit painstaking. It just occurred to me by looking at the directory structure that there was a correct place for everything to go, so I just shifted it there. At the time I had 4 mods downloaded and all of them in this wrong format. I guess things would have backfired very badly if I had made some mistakes in moving things about - BUT IT WORKED FOR ME
