Unity 5 Managed DLL considered Native

I have a DLL that I need to use to interface with a physical device. It comes in a few different formats - Managed with .NET 4.0, Managed with .NET 2.0, and Unmanaged (Unity's "Native"). I'm trying to use the Managed 2.0 DLL for my project, however Unity recognizes it as a Native Plugin, rather than the Managed Plugin that it is (or at least that I'm told). I can manually reference it in the Visual Studio solution, and it works just fine, but it still throws a compiler error saying that the namespace doesn't exist. Are there any workarounds or settings that need to be applied to use this Plugin with my project?