the model does not connect

@adanwaky,

It might be that the name of the model needs to have a capital first letter.
Instead of $this->usuario->ExisteUsuario() it might need to be $this->Usuario->ExisteUsuario
also...
instead of $this->load->model('usuario'); it might need to be $this->load->model('Usuario');

The reason I say this is because in your error message it states that... Call to undefined method Usuario::ExisteUsuario()
DID you notice the capital first letter of the Model?

Also, any reason why you don't return a value of FALSE if the model function if condition is not completed? I always try to either return a value or FALSE from a model so that I will always know if it was successful or not.