Well there is a few ways of getting it:
js code:
/* Looping through the contact list */
for ( var oContact = new Enumerator ( Messenger.MyContacts ); !oContact.atEnd(); oContact.moveNext() ) {
Debug.Trace ( oContact.item().Name );
}
/* If you know the email */
var oContact = Messenger.GetContact ( 'johndoe@hotmail.com' );
Debug.Trace ( oContact.Name );