Hello DSett. You can do it by adding new icon to the folder img\icons with name pinterest.png and adding that name to array in the php file \inc\social-media-widget.php, line 144 =>
$icons = array(
'facebook' => '',
'twitter' => '',
'google' => '',
'wordpress' => '',
'blogger' => '',
'yahoo' => '',
'youtube' => '',
'myspace' => '',
'livejournal' => '',
'linkedin' => '',
'friendster' => '',
'friendfeed' => '',
'digg' => '',
'delicious' => '',
'aim' => '',
'ask' => '',
'buzz' => '',
'tumblr' => '',
'flickr' => '',
'rss' => '',
'pinterest' => '',
);
This are changes for the parent theme, changes will be lost after theme update. In the child theme same widget can be created with new name. Function get_template_directory_uri() should be changed to get_stylesheet_directory_uri(), and folder img\icons copied to the child theme.