The XLL Add-ins window is a ToolWindow that lives inside Visual Studio, and lets you directly view and edit all of your add-in functions.
You can use the Add-ins window to navigate around your source code, and you can use it with the Visual Studio Properties window to make changes directly to your add-in functions without needing to pop up the XLL+ Function Wizard. You can select multiple functions, and make changes to them all at the same time.