To download and stream movies on Flixster you must create an UltraViolet account. This account will provide you with the rights needed to view your content on Flixster and beyond.
From UltraViolet's website:
When you purchase UltraViolet-enabled content, you get more than just a DVD or single-use stream of a movie or TV show. You also get additional content rights, which represent other ways you can enjoy your UltraViolet media. These can include the right to download a digital copy of a movie you purchased on Blu-ray. Or the right to create a physical copy (e.g. burn a disc) of a TV show you downloaded. Or the right to watch a movie on your smartphone (via streaming) when you' re on vacation while your DVD copy sits safely on a shelf at home. Please note that not all content will include the same set of rights, and that in some instances there may be a small fee to stream content that you have bought streaming rights for. For more information, or to contact customer service, please visit the UltraViolet website.