This article explains that Unitrends does not directly support archive to optical media, but provides a work around.
Optical media is not directly supported. It is possible to burn an archive to a CD, but first you need to archive to hard disk. Once there, you can make an image of the disk. Restoring the archive would follow the reverse process. For information on the types of supported archive media, see Archive media types in the Recovery-Series and UEB Administrator's Guide.