Hi Susan,
There was a Java bug related to accessing DFS share files with the Java file dialog, as explained here:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7012783
The OpenCDISC Validator itself has no known intentional restrictions that would prevent you from accessing files on a DFS share, so hopefully updating your Java runtime to 1.6.0_26 or later will resolve the problem.
Regards,
Tim