Using a C file as a plugin

I'm having some issues trying to make a C file able to be referenced from in Unity. I have compiled the C file into both a .so and .dll file, placed them in Assets, Assets/Plugins, Assets/Plugins/Android, and the same folder that the Javascript file I'm referencing the C library from is located. All tested on separate occasions, and all haven't worked, and always brought up the same DllNotFoundException.

Within the Javascript file I am referencing to the library using the following format:

    @DllImport ("libMYLIB.dll")
    static private function doStuff(foo : String) : float[] {};

    var results = doStuff(myString);

Is this the correct way to reference the library?