[Unity]Scene 'IntroScene' (-1) couldn't be loaded because it has not been added to the build settings

An error in in console:
Scene 'IntroScene' (-1) couldn't be loaded because it has not been added to the build settings or the AssetBundle has not been loaded.
To add a scene to the build settings use the menu File->Build Settings...
UnityEngine.Application:LoadLevel(String)
GameManager:StartMain() (at Assets/Scripts/GameManager.cs:72)
UnityEngine.GameObject:SendMessage(String)
Send:OnMouseDown() (at Assets/Scripts/Send.cs:10)
UnityEngine.SendMouseEvents:DoSendMouseEvents(Int32)


The cause of the error:  IntroScene to use in method StartMain()  doesn't add to  build settings menu.

public void StartMain() { Application.LoadLevel ("IntroScene"); }

How to fix : move to Menu> File->Build Settings... The IntroScene in Project view drags and drops to the screen of Build Settings.

It's just simple.