In many cases, the VSTO Runtime is installed automatically when you install Visual Studio or Microsoft Office. However, if you are a user trying to run a specific add-in and encounter an error, you may need to install it manually:
Before installing the VSTO 2010 Runtime, ensure your system meets these criteria: microsoft visual studio 2010 tools for office runtime -x64-
Managed assemblies that facilitate communication between your custom solution and the Office application. In many cases, the VSTO Runtime is installed
The Visual Studio Tools for Office (VSTO) Runtime is a set of components that allows Office applications like Word, Excel, PowerPoint, and Outlook to host the .NET Framework Common Language Runtime (CLR). This enables developers to create managed add-ins and document-level customizations that extend the functionality of the Office suite. The runtime includes two primary parts: This enables developers to create managed add-ins and
Conversely, if you have 32-bit Office installed on a 64-bit Windows system, you would actually need the x86 version of the runtime. System Requirements and Prerequisites
A set of unmanaged DLLs used by Office applications to load the runtime and your specific solution. Why You Need the x64 Version