I face subjected problem while trying to learn ADO.Net Entity Framework in visual studio. After a bit of searching, I come to know that I have not included EntityFramework.dll reference in my project. Just add the DLL reference and I hope that your will out of this problem.
Please note that in case if you don't find EntityFramework.dll in references then you can locate it in the project bin (Debug\Release) folder where you add ADO.Net Entity Data Model.
I hope that this will work for you as it worked for me. So please leave a comment if this post help you so other visitors can also trust on this solution.