How to use custom defines?

I setup the custom define named DEMO in the player settings but I am not sure how it works.
For example:

#if DEMO

	GUI.Label ( Rect ( 212, 200, 600, 50 ), "Disabled for this demo.", "menuHeadingStyle" );

	// otherstuff

Now how do I tell the compiler its the demo?

If you write DEMO into “Scripting Define Symbols” box of the Player Settings - Other Settings, then the part within the #if DEMO block will be included with your game.

If you erase the text DEMO from “Scripting Define Symbols” the // otherstuff will happen instead.