Unable to locate the specified class: Session.php [SOLVED]

Am integrating Facebook login using this library.
However after redirection from FB I get an error:

Unable to locate the specified class: Session.php

Other codes with 

PHP Code:

$this->session->set_userdata($str);
//and
echo $this->session->userdata('var'); 


seem to work elsewhere.

I think the problem is involved in this method but I can't find any problem.

Any suggestions?