Set the Root ID to the person in your database you wish to be placed at the root of your D-List. Set the Ancestor ID to the ancestor of that person you wish to link to.
If you are not sure what the ID of your Root/Ancestor Person is, you can view your GEDCOM file with any text viewer and search it for their name. GEDCOM files delimit surnames with slashes so if you were looking for "John Smith", you would need to search for "John /Smith/". Once you locate their name, ie "1 NAME John /Smith/", look for their level 0 record in the lines preceding it. It should look something like this: "0 INDI @XXXX@", where XXXX is their ID and should look something like like "I1" or "P-1000234".