I also get a 403 trying to load what you've linked, but dosbox_13.nmf loads fine and so do the games.
It determines the filename based on the major version number in the useragent - it would probably work fine if your browser reported chrome 13.something.
I wonder the server is refusing this request if the user-agent is not in their white list?
Or perhaps they are limiting it based on Country due to Copyright Issues?