Where can I find Microsoft.Office.Interop.Word.dll (2010)?

Install via nuget Microsoft.Office.Interop.Word.

If you have Office 2016 installed, you can get the file Microsoft.Office.Interop.Word.dll here:

C:\Program Files (x86)\Microsoft Visual Studio\Shared\Visual Studio Tools for Office\PIA\Office15\Microsoft.Office.Interop.Word.dll

Now that Visual Studio 2019 is out, you can install Microsoft Office interop libraries as part of an optional bundled component called Visual Studio Tools for Office (VSTO).

Microsoft have made this super easier, and you don't need to reference them in the GAC!

You shouldn't be searching for the dll on your local system yourself if you installed the assemblies correctly. See following link for information on how to download and install office interop libraries without installing office. Second link details how to add the assemblies to your project correctly.

Install Office Primary Interop Assemblies

Office Primary Interop Assemblies

For a further reference here are some pictures detailing how to add the dll correctly:

In your project, right-click on "References" and select "Add" and then "Reference".

Next select "Extensions" in the Reference Manager, scroll to find the correct dll. Which for Microsoft.Office.Interop.Word.dll Office 2010 is the version 14 one.