Quick Function: Search Custom Fields From wp-admin as per http://rogerstringer.com/2012/10/14/quick-function-search-custom-fields-from-wp-admin/

gistfile1.php

<?php
class post_search_meta {    
    function post_search_meta() {
        add_filter( 'posts_where' , array( $this , 'where' ) );
        add_filter( 'posts_join' , array( $this , 'join' ) );
        add_filter( 'posts_groupby' , array( $this , 'group' ) );
    }
    function group( $groupby ) {
        global $wp_query, $wpdb;
        if( empty($groupby) )   $groupby = "{$wpdb->posts}.ID";
        if( !is_admin() || !$wp_query->query_vars['s'] )    return $groupby;
        $mygroupby = "{$wpdb->posts}.ID";
        if( preg_match( "/$mygroupby/", $groupby )) {
            return $groupby;
        }
        if( !strlen(trim($groupby))) {
            return $mygroupby;
        }
        return $groupby . ", " . $mygroupby;
    }
    function where( $where ) {
        global $wp_query, $wpdb;
        if( !is_admin() || !$wp_query->query_vars['s'] )    return $where;
        $s = $wp_query->query_vars['s'];
        if( strpos( $where , 'AND (((' ) != false &amp;&amp;  strpos( substr( $where , 10 ) , 'AND (((' ) == false ) {  
            $where = str_replace( 'AND (((' , "AND ((({$wpdb->postmeta}.meta_key LIKE '%$s%') OR ({$wpdb->postmeta}.meta_value LIKE '%$s%') OR (" , $where );
        }
        return $where;
    }
    function join( $join ) {
        global $wp_query, $wpdb;
        if( is_admin() &amp;&amp; isset( $wp_query->query_vars['s'] ) ) {
            $join .= " LEFT JOIN " . $wpdb->postmeta . " ON " . $wpdb->posts . ".ID = " . $wpdb->postmeta . ".post_id ";
        }
        return $join;
    }
}
$post_search_meta = new post_search_meta();
?>