I used this description to get the nightly build of ASP.NET MVC for my project
http://blogs.msdn.com/b/henrikn/archive/2012/06/01/using-nightly-asp-net-web-stack-nug
I experienced exactly the same issue. You need to use the Symbols hosted on MyGet. Please see:
http://blogs.msdn.com/b/webdev/archive/2012/08/30/getting-symbols-and-source-with-asp-net-nightly-nuget-packages.aspx