I am using Visual Studio 2013 pro and Unity 4.6.
How to fix this issue?
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The reason that this occurs is because Visual Studio can't find the dll files for Unity without opening a specialized solution file. This file can be created using UnityVS, which can be found as "Visual Studio Tools for Unity" in the Visual Studio gallery. After installing this (make sure you have Unity closed when you do), open Unity, and a new tab will be present called "Visual Studio Tools". Click this, and click the button that says "Generate solution file" or similar, depending on the version of Unity and UnityVS that you are using.
Thanks to @IcyDefiance in chat, I can now say that UnityVS definitely does not support UnityScript or Boo, as said on their website. This is since version 1.9.x was released.
C# only: C# being the predominant language in the Unity ecosystem - the new Sample Assets are in C#, the Unity documentation will default to C# - we removed our basic support for UnityScript and Boo to better focus on the C# experience. As a result, VSTU solutions are now C# only and are much faster to load.