Without having a sample xml file, it's impossible to try to debug that plugin and see where it is failing. Tried to search if some innocent soul left a file hanging on the internets, but nope.
But one can see what others are saying: The nightmare that is Disqus export, and import into WordPress.
September 20, 2011
Disqus should have made this easy by using the generic WXR format, but
they haven’t, they went with their own schema. I’m out to prove this
kind of import can be done. It’s personal now. Share your Disqus
horror stories, or WordPress database-hacking stories here. Who knows?
Maybe you’ll have a solution I haven’t thought of.
And maybe, if it works as of now, adapt the following Disqus Comments Importer Script in PHP to route the output into a
January 12, 2012
Recently I moved from Disqus to my own custom commenting system for
several reasons like more control over comment notifications to
authors, showing comments count anywhere easily, etc. So I had to
figure out a way to move about a hundred comments from Disqus's system
to my own Database. Googling did not help me find any good script for
such a requirement neither did Github Search yield anything. So, I
decided to give it a shot, took some time as I had to do it with
http://php.net/dom and had no previous experience but overall it
wasn't more than a day's task and most of it was a breeze. I would
like to share the code here just incase it helps others!