Please follow the below steps to open the SGML file and see if that works :
Launch FM 2015 in structured mode.
Go to Structure Tools > Edit Application definitions.
structapps file will get open.
Now under Structure View we will see StructuredSetup and Version.
Now Open Element Catalog.(Element>Element Catalog).
Under Structure View if we put a cursor below version then under Elements catalog we will see SGML Application and XML Application.
Select SGML Application and select insert.
Once we insert SGML Application element, then we have to give Application name.(say mysgml).
Press Enter(or select from element catalog).
Now select Entities
Press Enter and insert Public element.
The Public element has two children: <PublicID> and <FileName>. In <PublicID>, mention the Public ID as mentioned in the DOCTYPE statement. In <FileName>, mention the complete system path of the corresponding DTD file to be used (downloaded & kept there). This needs to added for the various Public IDs that may get referenced.
Save the file and use 'File->Structure Tools > Read Application Defintitions' to process it.
Now, open your SGML file in FM.
Choose the appropriate application (mySGML, as set in step-2 above) from 'Use Structured Application' dialog.