As of today—November 2nd, 2015—the Orsinium DLC for The Elder Scrolls Online is available for players on PC and Mac. As you can see here, the Xbox One version is planned to be released on November 17th and the Playstation 4 version on November 18th.
Those with an active ESO Plus™ membership will automatically have access to the Orsinium DLC game pack. Otherwise, the DLC can be purchased for 3,000 Crowns via the in-game Crown Store. Additionally, the DLC's Collector's Edition contains the Orsinium DLC game pack itself as well as miscellaneous bonuses for 5,000 total.
The DLC adds a number of new quests, group bosses, delves, public dungeons, new items, and even a new game mode, as well as numerous other new features and several smaller optimizations. For more information, see the official news update on the DLC.