Silk Test Workbench will not reference the Microsoft Office Primary Interop Assemblies
Although the Microsoft Office Primary Interop Assemblies are stored in the GAC (Global Assembly Cache), they are not true dotNet assemblies. They are a COM (common object model) structure that has a dotNet wrapper around them to allow storage in the GAC. Consequently, to use the Interop Assemblies in Silk Test Workbench you must copy the required interop to another, non-GAC, folder.
However, using Windows Explorer to navigate to the GAC causes Windows Explorer to load a custom shell specifically for viewing the GAC. You won't be able to copy/cut/paste/delete from within this shell. The solution is to navigate directly to the GAC from the Run window on the Start orb. Enter the following command into the Run textbox:
You will now be able to copy your required Interop Assembly to a new folder where it can be referenced from Silk Test Workbench.
This does not affect Silk4Net. Visual Studio has the built-in functionality to reference the Interop Assemblies natively.