If you wish to take the side of the Free Peoples, your player character must first attain level 40. Once you are at least this level, visit one of the stable-masters that can take you to the Ettenmoors. One such route can be found at Elrond's stables in the western section of Rivendell, just west of the Trainer area. Speak to stable-master Remros about the Ettenmoors, and he'll send you on your way.