There are two kinds of add-ins that Word can use.
1. COM add-ins.
These are .EXE or .DLL files. They can be made available by using COM Add-ins from the Tools menu.
NOTE: COM Add-ins is NOT on the menu by default. If you do not see COM Add-ins under Tools, you must add it. Type "COM" into Word Help and you will find instructions on how to add it to the menu. Essentially, you customize the Tools menu.
Once it is added, you simply select COM Add-in from Tools, and then click the Add button and browse to the folder with the file.
Obviously, as these are either .EXE or .DLL files, they are compiled, and since you mention C# I am assuming this is what will apply for you.
2. Templates and Add-ins...
These are the native "add-ins" that Word understands. They are .DOT (template) files that act as code containers. Being .DOT files they are NOT compiled and must be parsed by VBA. However, because no compiler is required (VB, C++, C#) they are much easier to create. Further, these global templates can be dynamically loaded and unloaded in real time, either manually (using the menu) or via VBA code itself.
Global templates (.DOT add-ins) do NOT pass on Styles. They are code containers. Once listed and loaded - these are different - all procedures (Subs and Functions) become available to any document.
Gerry
