The "netdisaster.txt" file is a mere text file.
Upload it at the root of your site.
It can be empty or not.
Netdisaster will try to access this text file when a URL points to a disaster, in the form of:
http://www.netdisaster.com?go.php?mode=disaster&url=http://www.target-site.com/directory/any_page.html
In the example above, the netdisaster.txt file has to be located at:
http://www.target-site.com/netdisaster.txt