Ultimately "Document" is defined in the COM API type library "roamer.tlb". Visual Studio normally generates the Interop dll for you when you add a reference to a COM component in a .Net project. The only reason that Interop.Navisworks8.dll was included with Navisworks 2011 is that another Navisworks component was using the COM API. That component was rewritten to use the .Net API for Navisworks 2012, so we no longer needed to include the Interop DLLs.
To solve your immediate problem, you can generate your own copy of the required interop DLL. Right click on references in your visual studio .net project and choose add reference. Go to the COM tab and select "Navisworks Automation API Library 9". Visual Studio will create an Interop DLL called Interop.NavisworksAutomationAPI9.dll and add a reference to your project.
The same functionality is available via the Navisworks .Net API in the Autodesk.Navisworks.Api.Automation.dll assembly.
Tim Wiegand Software Architect Navisworks Autodesk, Inc.