If you are receiving error like in title, then add in the reference :
EntityFramework
Search for:
EntityFramework.dll
In my case it was in folder:
C:\Program Files (x86)\Microsoft ASP.NET\ASP.NET MVC 4\Packages\EntityFramework.5.0.0\lib\net40
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Or you can do it with NuGet. Go to tools -> Library Packager Manager -> Manage NuGet Packages for Solution:
Then search for EntityFramework and install it: