The EntityFramework package is not installed on project
Solution 1:
Hopefully no-one is as stupid as me, but for the benefit of searchers:
One possibility for this error - In the Package Manager Console, there is a dropdown for 'Default Project'. If this is set incorrectly, you will get The EntityFramework package is not installed on project 'x'. Change the dropdown to your EF project and all is good again.
Solution 2:
In my case, restarting the Visual Studio helped.
Solution 3:
Just update NuGet to 2.x. EF 5.0 requires it.