One of the things that Firaxis basically nailed about Civilization V was mod support in single-player. It’s a big, shiny button on the bottom-left of the screen, labeled “Mods”.
For the first time, though, there’s a button in the single-player menu that’s alsoavailable here. The same options as in the single player setup screen are available on the next page, including the new “Online” game speed, which doubles the game pace and is intended for multiplayer.