This feature is available only to Pro stories. To get Media features for Starter stories use the Media feature on the Character Nodes
Once a media node has been created in the graph, click 'Select from Library' to start.
Clicking the 'Select from Library' button will bring up the Media Library which contains all your video files, audio files, and images. The Media Library is also accessible from the left-hand side menu by clicking on 'Media'.
Once you open the Library, click on 'Add Media', which will open the Uploader. Once you selected or dropped the file(s), click upload. When the upload is finished you will see a '✓' icon on each uploaded file, then you can close the Uploader and select the file which you would like to add to the Media node.