I found the problem, the data must be passed as follows in the config file (pagination.php) within:
$config['full_tag_open'] = '<div id="pag"><ul>';
$config['full_tag_close'] = '</ul></div>';
$config['prev_tag_open'] = '<li id="prev">';
$config['prev_tag_close'] = '</li>';
$config['next_tag_open'] = '<li id="next">';
$config['next_tag_close'] = '</li>';
$config['first_link'] = 'Home';
$config['first_tag_open'] = '<li>';
$config['first_tag_close']= '</p> </ li>';
$config['last_link'] = 'Last page';
$config['last_tag_open'] = '<li>';
$config['last_tag_close'] = '</p> </ li>';
$config['num_tag_open'] = '<li>';
$config['num_tag_close'] = '</li>';
$config['cur_tag_open'] = '<li><strong>';
$config['cur_tag_close'] = '</strong></li>';
and the controller:
$this->load->library ('pagination');
$config['base_url'] = $url;
$config['total_rows'] = $all_reg;
$config['per_page'] = $amount_pag;
$config['num_links'] = $num_links;
$this->pagination->initialize($config);