The Wii U Netflix app's launch, like the console's launch itself, didn't exactly go smoothly. The Wii U Netflix app somehow didn't ship with the '@' function for password entry.
A Reddit user reported the issue to Netflix support. The only solution thus so far is for users with the @ symbol in their password to change their password via the Netflix website to something else that's actually available on the app keyboard.
Neither Nintendo nor Netflix have released a statement as to why or how this could've happened. Though the issue isn't so serious that it requires an urgent fix, it's definitely quite an inconvenience for Netflix users.