I have had similar problem on Ubuntu, except that it was even worse there:
https://bugs.launchpad.net/ubuntu/+source/qdvdauthor/+bug/216309
This is what happens. If I click on Add/Organize videos, I cannot really add them to the project. It behaves better than on Ubuntu in a sense that there is no “Error” message, I can see the thumbnails, but I cannot do anything with them, except to play them with a right click button.
The only way I can add videos to the project is to right click on the left area of the project space and without giving you any option you get to chose which files you want to add. Or maybe this is how the program is supposed to work?