The 'Invisible Kingdom' is considered as DLC therefore you must have chosen Hoshido or Nohr in order to puchase the third route.
The third scenario is available as a 2,000 DLC. As previously mentioned, all three scenarios of Fire Emblem Fates are said to have about the same amount of content as Fire Emblem Awakening. http://www.siliconera.com/2015/07/08/take-a-look-at-fire-emblem-fates-third-scenario-invisible-kingdom/#fWADEkWEh8i80lyQ.99
Two of the three scenarios are available in the two different Fire Emblem Fates versions while the third is only going to be available as DLC.
http://www.geeksnack.com/2015/07/09/fire-emblem-fates-dlc-scenario-invisible-kingdom-receives-new-trailer/
Unlike the Hoshido and Nohr campaigns, the Invisible campaign is only available digitally as DLC. To purchase it, you can select the “Pave a new future” option, which is the bottom-left option before loading a save file.
http://serenesforest.net/2015/07/09/fates-the-3rd-route-invisible-kingdom-now-available/