The demos are taken care of by the challenge-tv site (thanks dK).
To submit demos, you should first convert the demo to .QWZ format. Do not submit .ZIP of .QWD, they will be deleted. After you've converted the demo to .QWZ, check that it is renamed properly, ie. winner-loser_map.qwz.
After you've done the necessary steps, e-mail it as an attachment to email@example.com and write Duelmania to the subject line.
You may also dcc the .QWZ demo to chtv|dK in irc, if you feel it's a better way.