When you install Qt’s msvc2015 libraries, you need Visual Studio (Express will do) with the C++ development tools included. They are deselected by default, probably because of the immense download size. 
Then Qt Creator should be able to auto-detect it as a compiler.
You don’t need to follow the instructions you mentioned, as this is only required if you build Qt itself from source.
If you just need those parts, simply right-click and export them, then import them into the old Fritzing. This will cause a little trouble when you upgrade though, as you will then have two parts with the same module id. You will then need to physically delete the imported one again (at least the .fzp), or change its module id.