XML DTD for downloadable stories
There's an unmatched ( in the declaration of the contents of story.  I recommend that you actually test the DTD using a validating parser such as OpenSP before posting code for review.

6.1 is not satisfied: you have declared content for story but no attributes.

6.2-6.4 are not satisfied: the url, title and preview elements must each appear exactly once.

6.6.1 is not satisfied: you only allow content before video and not video before content.

7.1 is not satisfied, because title has been declared as an empty element.