Hello,
I'm guessing you're trying to open that C++ add-in that is highlighted.
However, unlike for Python, for C++ the development environment is set by default to Visual Studio for Windows and to XCode for macOS. This is because of how C++ add-ins are created, see OS Specific Issues article for further info .
So when trying to edit a C++ add-in through Fusion, you need to have one of those installed on your PC depending on your OS. Otherwise Fusion might prompt you to install it or you will have to go directly into the folder where the add-in is located and open it there.