Through the server, the option to create a default content through the schedule doesn't exist. It exists only with premium S/I scheduling. I'm not sure but you can check (I'm sorry I don't have a DBD screen accessible right now to check with you) through the menus of the screen itself (Magicinfo menu when you press home on the remote or the Menu of the screen itself) if there is a default content choosing. I've done it but on the old MD screens so I can't remember where this option exist in the new DB screens.
Also note that the default content can't be for example an image or a video, but it must be an SWF file which can be made by Adobe flash professional, when a customer asks me to create a default content and provide me with only an image I add this image to adobe flash professional and create a fading effect every 5 mins to save the screen from color burn.