Melanie is Charles Widmore's secretary. She tried to stop Desmond from barging into Widmore's office, but Desmond ignored her. She apologized to Widmore for being unable to stop Desmond but Widmore told her it was okay and ordered her to leave the room. ("Jughead")
This character is officially credited as secretary in the ABC Medianet press release for "Jughead". Her name was revealed in dialog between Charles Widmore and the secretary.