Hi riverratt,
Welcome to Laptop GPS World.
The full-screen mode is available only when both of the following conditions have been met:
- Route to your destination has been set
- 'Provide Driving Guidance' has been enabled in the GPS Task pane
But the fact that you are asking this question is an indication that this feature is not as user friendly as it could be. I wish full-screen mode was available at all times - regardless of what other features have been enabled.