If it's an MY10 (i.e. WM series 1, not series 2) there will be a DVD player in the glovebox that takes a maps disc (quite expensive to buy if you don't have one). If there's no DVD player in the glove box, then you don't have Nav.
If it's a WM Series 2, it will have Holden IQ (touchscreen system). Navigation is built in with these and it should work by pressing the NAV button.....