I believe Entity Framework 6 is intended for .NET 4.5. I'm wanting to use it for a Visual Studio 2010 project, so .NET 4.5 isn't available.
Does it support .NET 4.0 and hence Visual Studio 2010?
Entity Framework 6 does support .NET 4.0. You should update your version of NuGet then try again. You can find the download at http://www.nuget.org/. I successfully downloaded EntityFramework 6.0.0-rc1 from the main nuget.org feed from an application targeting .NET 4.0