I believe you do not have NuGet Package Manager extension installed.
If not, you can click TOOLS -> Extensions and Updates… -> Online, select NuGet Package Manager, and click Download:
After it downloads, you will be prompted for administrative elevation, and then to restart Visual Studio.
Tested in: Visual Studio Ultimate 2012 (Version 11.0.60610.01 Update 3)
UPDATE: I’ve just came across this problem in Visual Studio 2012. So despite some comments here, it is not necessarily installed by default.