How to use unmanaged DLLs in SharePoint project?

I am using SPSS .NET interop library to export data to SAV file.

I get error

System.TypeInitializationException was caught HResult=-2146233036 Message=The type initializer for 'Spss.SpssThinWrapper' threw an exception. Source=Spss TypeName=Spss.SpssThinWrapper

InnerException: System.DllNotFoundException HResult=-2146233052 Message=C:\inetpub\wwwroot\wss\VirtualDirectories\80\bin\win64\spssio64.dll not found.

Sharepoin can't found external dll although it exists in that location.

How can I use this kind of libraries with SharePoint projects?

Any advice or suggestions will be highly appreciated