The basic idea is that there is a filesystem browser and more specialised views for video, audio, and images. In the filesystem browser, you can add a folder to your library. The first item in a folder list represents the folder itself. There is a button you can press for opening a little menu with buttons for "add to library" and "enqueue to playlist". Folders added to the library are being watched and their contents appear in the specialised views. By default, the library contains only the internal media folders of your device (where the Nokia example files are).
You can manage folders in your library by going to the "library" view in the filesystem browser (button on the right). There you can select what kind of media MediaBox should watch in the folder, or you can remove the folder from the list of watched folders again.

I agree that this whole concept is a bit hard to discover for yourself, and some kind of first-steps tutorial for MediaBox would be a good thing.