I can only get the numpad plus to work, using sf::Keyboard::Key::Add.
The standard plus key reports a key event, but with id 0.
For the - there are two entrys in /usr/include/SFML/Window/Keyboard.hpp
but for + there is only one.
This problem occurs on Arch Linux (sfml 1.99) and Ubuntu (sfml git)
Keyboard management has to be improved to avoid such problems: