The Collaborative International Dictionary of English v.0.48:

Amiableness \A`mi*a*ble*ness\, n.
   The quality of being amiable; amiability.
   [1913 Webster]

WordNet (r) 3.0 (2006):

amiableness
    n 1: a disposition to be friendly and approachable (easy to talk
         to) [syn: affability, affableness, amiability,
         amiableness, bonhomie, geniality]