I am not really sure, but as far as I know Rhino 5 is .NET 4.0 and Rhino 6 .NET 4.6 (?) . And as far as I know those Office Tools for Visual Studio which include the .dll you are referencing exist also in various versions (11.0, 12.0, 14.0, 15.0), which are probably compiled for different .NET Framework versions.
So maybe the reason why the .dll doesn’t load is a compatibility issue ?!
Other reasons why the .dll fails to load could be maybe the file is not where it is supposed to be (“V:\Fiches something”) ? Or missing dependencies…