It's an XML document, right? So the reference to the DTD tells the parser to use that DTD to validate the document. And as you know, those characters you inserted change that DTD reference into a comment. So
you should be able to figure out for yourself what they did.
By the way your real problem is probably that RAD can't find that DTD. Most likely you are running it in an environment where you have to go through a proxy server to get to the internet and you haven't configured RAD to tell it where that proxy server is. (Of course that's a guess, you still haven't said what the error message is, but I think it's likely.)