New to CI (be kind)

[eluser]Bart Mebane[/eluser]
@k2zs: You need to load your model before calling it.

Code:

$this->load->model('subNav_model');

You can do this either in your controller constructor (if you're using the model in all or most of the controller functions), or inside a specific controller function. I don't know if the model code you posted was the entire file, but a model, unlike a view or a helper, needs to be defined as a class.