Sorry to necro this thread - I was also trying to solve the problem with the datafile.dtd hosted on the long-dead logiqx.com.
Using the Internet Archive, I was able to find an old copy of the DTD file from 2023 here.
I thought maybe changing the URL in the DAT accordingly would do the trick, like so:
<?xml version="1.0"?>
<!DOCTYPE datafile PUBLIC "-//Logiqx//DTD ROM Management Datafile//EN" "https://web.archive.org/web/20230405091520/http://www.logiqx.com/Dats/datafile.dtd">...but then I get an error saying:
Expected DTD markup was not found. Line 1, position 3.
Next attempt: Download the DTD file and use it locally (put it in the same folder as the DAT):
<?xml version="1.0"?>
<!DOCTYPE datafile PUBLIC "-//Logiqx//DTD ROM Management Datafile//EN" "datafile.dtd">...and that works! 
Hope it's able to help someone.
I have a question in return: can this tool also generate Redump-style CUE sheets if I provide the correct DAT? So far all I've been able to get it to do is write GDI. Maybe I am misunderstanding the instructions...