I have added a filter field, so you can display only certain record, like the active ones or from a group only, so in your definition file (.yml) put something like this:

    moneda:

        class: DbDropDown

        params:

            field:valor

            table:catalogo

            primary_key:cod

            filter: where grupo='MONEDA'

    pais:

        class: DbDropDown

        params:

            field:valor

            table:catalogo

            primary_key:cod

            filter: where grupo='PAIS'

So the source modified is like this:

<?php if (!defined('BASEPATH')) exit('No direct script access allowed'); 
include_once("dropdown.php");

class DbDropDown extends DropDown{
    var $primary_key = 'id';
    var $table = '';
    var $field = '';
    var $filter = '';
    function DbDropDown($name,$params){
        DropDown::DropDown($name,$params);
        if(isset($this->params['primary_key']))
            $this->primary_key = $this->params['primary_key'];
        if(isset($this->params['filter']))

            $this->filter = $this->params['filter'];

        if(isset($this->params['table']))
            $this->table = $this->params['table'];
        else
            show_error($this->lang->line('codexforms_dbdropdown_table_not_defined'));

        if(isset($this->params['field']))
            $this->field = $this->params['field'];
        else
            show_error($this->lang->line('codexforms_dbdropdown_field_not_defined'));
    }
    function prepForDisplay($value){
        if(empty($value))
            return '';
        $CI = &get_instance();
        $CI->db->select($this->field);
        $CI->db->where($this->primary_key,$value);
        $query = $CI->db->get($this->table);

        $result = $query->result_array();
        if(count($result) == 0)
            return '';
        else
            return $result[0][$this->field];
    }

    function getList(){
        $CI = &get_instance();
        if(!isset($CI->db)) $CI->load->database();

                $html = "<option value =""></option>";
        $field = $this->params['field'];
        $table = $this->params['table'];

        $result = $CI->db->query("SELECT ".$this->primary_key.",$field FROM $table ".$this->filter);
        foreach($result->result_array() as $row){
            if($row[$this->primary_key] == $this->value)
                $html .= "<option value="".$row[$this->primary_key]."" selected>".$row[$field]."</option>n";
            else
                $html .= "<option value="".$row[$this->primary_key]."">".$row[$field]."</option>n";
        }

        return $html;
    }
}

?>