If you Google 'Unable to load the requested class' you will get a lot of returned results specifically related to CodeIgniter that may be able to help you track down the problem.
From what I can see as the general cause is that it may be something to do with the letter casing of some of the library or model files.
Other than that, trying to autoload the library (Which is not currently possible with flexi auth) seems to return the same error.
Have you renamed any of the library files?
Are you loading the library similar to the following method?
Code:
$this->auth = new stdClass;
$this->load->library('flexi_auth');
