If you want the LCG in Offline mode, youll need to enable it in Arena Settings first.
On your clipboard, go to OPTIONS > ARENA SETUP > ENVIROMENT > Arena Settings. Find FlightModeFlag on the list and click CHANGE. Now check the box Lead Computed Gunsight - OK.
In that same list you can also change PlaneAmmoCount and FuelBurnRate for unlimited fuel and ammo (great for gunnery practice!).
Clicking on SAVE willsave the Arena Settings for the next time you come in.
Now when in flight, press TAB to toggle LCG between targets. If you want to target a friendly, make sure Friendly Lock is enabled (Toggle with CTRL_TAB).