The issue is happening because you might be having multiple versions of entity framework packages used in the project.
Verify your packages.config of the project in which you are getting this error. It may be using entity framework version 6 beta, If so, remove that config entry from the package configuration and also remove the EntityFramework dll reference from the project.
If you want to use Entity Framework 5.0, then you have to choose the “Stable Only” option when you add EntityFramework reference through Manage NuGet packages as below. If you have chosen the “Include Prerelease” option, then EntityFramework 6 beta version will be added because that is latest version.
So, add a new reference with the “Stable Only” option then try the enable migrations command, it should work fine.