The issues on the North American and European PC/Mac megaservers have been resolved at this time. If you continue to experience difficulties at login, please restart your client. Thank you for your patience!
Like every other expansion for one year or so following their release, they are a separate thing. You need to buy it, along with the base game, to play it. After about a year or so, they'll be antiquated and turned into "DLC", which means you can buy it with crowns, gift/sell to other players and access it with ESO+.