I believe that you have not installed the NuGet Package Manager extension.
If not, you can click TOOLS → Extensions and Updates ... → Online , select NuGet Package Manager and click Download :

After loading, you will be asked to increase the administrative height, and then restart Visual Studio.
Tested in: Visual Studio Ultimate 2012 (version 11.0.60610.01 Update 3)
UPDATE I just stumbled upon this problem in Visual Studio 2012. Therefore, despite some comments here, it is not necessarily installed by default.
Bhavesh Chauhan Nov 13 '12 at 7:29 2012-11-13 07:29
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