[eluser]jvicab[/eluser]
make sure the name of the file is usermodel.php and
also you are assuming you create a constructor but you don't. Actually you have a diferent function, I mean, __User is not a class constructor, it is just another functuion inside it. Contructor name should be the same name of the class (function User), or better (php 5) function __construct(). It should be like:
function __construct() // instead of __User
{
parent::__Controller();
$this->view_data['base_url'] = base_url();
$this->load->model('Usermodel');
}