MusicBox.NET - Playlist - Add From Folder
If you want to add all the files in a folder to the playlist then this is the best option instead of selecting all of it using the "Add Media" function. You can do so by either pressing CTRL + SHIFT + F on the main window or click the "Folder" button available in the playlist window, a folder browser should pop up, navigate to the folder you wanted to add to the playlist and press "OK"