Sorry that functionality was removed from the later themes since it caused more confusion to the users than really helped them. If you want to replicate the same, you can modify the function mo_site_logo() in framework/functions/header-title-area.php line number 6. Output a custom retina logo img element there at the end of the function –
$output .= '<img class="retina-logo" src="http://yourretinalogo.png" width="120" height="240" alt="Retina Logo"/>';
and then use custom CSS –
#site-logo a img.retina-logo { display: none; }
@media only screen and (-webkit-min-device-pixel-ratio: 2) {
#site-logo a img.retina-logo { display: block; }
#site-logo a img.standard-logo { display: none; }
}
You can also refer to some help on the net.