Are you trying to add it under Animal Crossing: City Folk (or Wild World)? Because both of those require a character/town to be linked to the code.
For NL, we don't have it listed as a seperate game because it doesn't use its own friend code. It just uses the general 3DS friend code, which you can add under "Nintendo 3DS Code", though I see you've already done this, so you should be good to go!
"Without contraries is no progression. Attraction and repulsion, reason and
energy, love and hate, are necessary to human existence." ~ William Blake