How do I make my menu show when I start the game?

I have a Canvas with the scrip.
Nested into the Canvas are 4 GameObjects: Background, MainMenu, LoadMenu, Options
Inside of each is a ready build and working menu.
All start with the little box besides their name in the Inspector unchecked.
So when I press play in the editor only a black screen appears.

How do I make the MainMenu appear? (Aka: how do I check the box in the Inspector with C#)

Im now 3 hours and a number of videos and help files in, without any success.

I discovered that if I put script into the MainMenu and write

Code (CSharp):

1. gameObject.SetActive(false);

I can make it vanish, but the opposite doesnt work since the MainMenu isnt even ?instantiated? at game start.