When you click on “Choose file” kiwitrees hands over all functionality to your computer’s operating system. Kiwitrees has no way to know the folder structure of you local machine.
So which folder is first displayed there depends on how your operating system works. For me (Windows 10) it opens the last folder I used in the Windows file manager programme (“explorer”). I have no idea how your apple machine functions in this regard. But I am pretty sure there is no concept of a “default” folder.