How would you address the Pope?
Your Holiness is the proper way of addressing the Pope
A Cardinal would be referred to as, Your Eminence
This question was answered on the Vatican's website: Pope Francis does not have a direct mailing address but you can contact the Vatican's press office at email@example.com . Alternately you can mail the pope via usual mail at this address: His Holiness, Pope Francis PP. 00120 Via del Pellegrino Citta del Vaticano