explain how you got it working please
All right, since there aren't any clear tutorials, is basically what DeathChaos said:
1 - Install NTR CFW 3.3
2 - Choose the folder of the game (FE fates usa) and extract both folders (plugin, castle) on the root of the SD card
3 - Run NTR CFW
4 - And finally, play the game, make sure that you get a "Green screen" before starting the game.
Now, this may fail or not, so you'd want to try it several times until you get it working, you don't have to access "Amiibo" in order to activate it, the units will appear as soon as you enter "My Castle", take advice that these units (Marth, Ike, Lucina, Robin, Anna and that other DLC character) will join you automatically, but you don't get to fight them, a little bit of a bummer, but beggars can't be choosers, they also don't come with their weapons, so no Falchion or Ragnell in here, unless you use a certain cheat program to get them, or you get lucky in that dlc mission (And i'm not sure if those weapons appear in that chapter either...)
Last note, you also have to make sure you are in Ch.16 or beyond, if the "Patched" version doesn't work with you, try to use the original plugin instead.