In Visual Studio, if the XLL Add-ins window is not visible, you can use the View/Other Windows/XLL Add-ins menu to show it.
By default, the Add-ins window will appear on the same tab as the Solution Explorer. This is usually the most convenient place to keep it. It is a good idea it not share a tab with the Properties window, since the Add-ins Window and the Visual Studio Properties window work together closely.
A typical layout for the Add-ins window and the Properties window is shown below.