You may find that your MLDonkey bittorrent client cannot connect to private bittorrent site’s trackers. You would probably realize that MLDonkey is not one of the supported bittorrent clients on these private bittorrent sites.
What you can do is to edit BT-user_agent to other supported clients like utorrent (uTorrent/1700), azureus, etc.
To change your BT user agent, simply login to your MLDonkey adminstration page. Go to the Options. Under the plugins drop-down list, select BitTorrent and change the BT-user_agent field.