Dropbox works by synchronizing files that are stored on your computer to the cloud. OnSong reads the files directly from the Dropbox cloud so if they are not showing up in OnSong, it's because your files have not been synchronized from your computer to Dropbox. This can happen simply because it takes time to synchronize files. If the files never appear, it may be caused by the Dropbox desktop app not working properly. Make sure that you have a good Internet connection and that the Dropbox program on your computer is functioning. You can get help from Dropbox here: https://www.dropbox.com/help/desktop-web
You can also sign in to dropbox.com from a web browser to see the files and folders in your Dropbox as they appear to OnSong. Again, if the files are not present here, they will not be available to OnSong as the files have not been uploaded to your Dropbox account.