My libraries in autoload.php is:
autoload.php
$autoload[\'libraries\'] = array(\'database\',\'pagination\',\'session\',\'cart\',\'form_validation\');
Thanks to all,
finally i found a solution,
I loaded this library like this:
$this->load->library('Session/session');
I used it because in codeigniter 3.0 the libraries files of session is located in the path :
System/libraries/Session/Session.php.
Now, it is working...