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For online play you should keep an eye on the Doomsday Engine Builds, RSS feed. Each time a new multiplayer build is released that feed is updated with new download links.
The Doomsday Master Server can be found at http://dengine.net/master.php however you shouldn't need to enter this manually unless you are using a much older version (e.g., ver 1.8.6).
I can't say I've seen many people playing online with 1.8.6 now that the multiplayer testing is in full swing. Although Doomsday hasn't really been all that popular for multiplayer historically, for various reasons.
Most active games I'm seeing now are on our dedicated servers using the latest multiplayer test build.