Place the mod in the modules folder. Do not place within a folder already within the modules folder.
Your modules folder should be here...
E:\Program Files\Mount&Blade\Modules
And have a folder already within called Native.
Say you have a mod called Test. What you may have done is put it in the native folder already there, resulting in...
E:\Program Files\Mount&Blade\Modules\Native\Test
So the game doesn't recognize what you added.
Place the folder containing the mod within the modules folder, resulting in a path location like this...
E:\Program Files\Mount&Blade\Modules\Test
It should work then. And of course, your drive may be C: or D: or whatever you installed it to. And, if you did all this correctly, when you bring up the M&B menu, make sure you choose the mod you installed from the drop down menu there, and don't leave it on Native. Also, not every mod will have a drastically different main screen than native, or a different map or anything that you'll immediately notice.