I wish to burn a Photostory to a DVD-RW disc. Is there an option to leave the disc "open" to allow files to be added at a later date? I am using "Movie Edit Pro 2015 Plus" in Windows7. KD.
Since you are using Movie Edit Pro I have moved your post to the correct forum area.
The answer to your question is, No. MEP projects, once exported and burned to disc, require that the disc be finalised in order that a DVD player can read it. To leave a disc "open" is to treat it, essentially, as a Data Disc, which would then be playable only by a computer.
As Jeff has pointed out Movie Edit Pro (MEP) has to close the project but as you are using RW discs why not just save the project file and add to it later and then produce a new file to overwrite the disc?