Use this filter to add javascript below (after) the slider JavaScript call. {type} can be flex, coin, responsive or nivo.

function metaslider_nivo_js($javascript, $slider_id) {
    $javascript .= "alert('Nivo slider has loaded');";

    return $javascript;
}
add_filter('metaslider_nivo_slider_javascript', 'metaslider_nivo_js', 10, 2);