Project containing your Unity package will not build for Windows Store. Therefore in its current state it is not possible to use GameSparks in Windows Store games.
(Who thought it would be a good idea to make one Empty dll and rename it 4 times?)
More details can be found here:
It looks like the unity build process has changed slightly, as it used to copy DLL's based on the file name only (ignoring the assembly name)
I'm about to create a new set of DLL's and republish the SDK to get around this change.
Thank for pointing it out.
Does anybody knows if this problem was solved? Also could be great to know if this plugin works within Unity Free (Mobile deploy).
Yes this issue was resolved in newer versions of the SDK.
Please delete the SDK from your Unity folder and re-download the latest version.