Authorization with Dropbox was successful, and there is a Heroku subfolder in my Apps folder in Dropbox, but no application subfolder or files.
This might be caused by the use of Heroku Fork. If the content of your git repository is empty, it means there wasnt a deploy to the app using the Git service. The Dropbox sync system requires a git repo to be present on the Heroku side in order to work or the app to have never been deployed.
You'll need a git repository containing your application code, and then you can connect it to Heroku by running
heroku git:remote -a your-app-name from your app directory and then perform a git deploy by running
git push heroku master.
An alternative is deleting the app and recreating it.
Once you've done this one time you shouldn't need to again.