In your setup, turn off auto downloading and try again. See, if you have auto downloading on, it will ALWAYS try to download any mods that the server has, even if they are server side, even if they are superfulous, like a bot route for a map that's not being used! It's odd.
And if the server has auto downloading turned off, it will fail, and thus not let you connect.
But if what I suggested doesn't work, you may simply have to go to pcgamemods.com (or wherever), download dcmod and install. Though from the sound of it, it's probably not that great of a mod to have on the server you're joining anyway.