When adding a new project to a solution in Visual Studio 2013 and encountering an inability to reference other projects...

Well, you likely need to set the target framework for the project to be 4.5.1 instead of 4.5. Duh! The symptoms of this sickness will be a triangle with an exclamation mark in it appearing over the reference in the Solution Explorer and an inability on the part of the compiler to understand namespaces in the other projects you call out to.