Once you've created a player, you can assign one or more playlists to it, using the Media module. See Assigning Content to Players. If your player template doesn't accept playlists, you can assign a video to it using Quick Video Publish or assign content to the player programmatically by editing parameters in the publishing code for the player and specifying the video ID property of the video you want to play. See Assigning Content to Players Programmatically.