It may be that the files didn't properly download.
First, check to make sure it isn't a compressed file, like .rar or .zip. If it is a .exe instead, you can just install it to the Modules folder in the Mount&Blade folder. If you already installed the mod to the Modules folder, delete the mod files, and try decompressing and copying to 'Modules' again if you have one of the first two formats, or install the .exe again.
And if all of that fails, try deleting the files, and downloading it again. It may be that the download messed up or something.
And that's all the help I can give. Hope it helps.