D2L offers several ways for professors to provide feedback on student work submitted via the dropbox toolset. You can add a simple score and some general feedback, upload a marked up version of the student submission, or within Turnitin, provide line-by-line commentary on student writing. COLI now has a single, short tutorial video showing students how to access their grade and any feedback, however you supplied it, on their work.
Like any YouTube video, you can easily install this as a link or embedded player within your D2L course space.