Hi Scott,
Make sure you follow the steps in the .NET Languages Add-on's C# Language "Getting Started" documentation topic.
Do those things first, then add references like what you are doing. Make sure you are also adding any assemblies as references that your assembly requires as a dependency. Otherwise some types might not be able to be resolved. Maybe that's what's happening here? Doing the reference adding is the same concept as adding a reference to a source project in Visual Studio.
Once you do that, the namespaces and types within the referenced assemblies should be available for use. Just like any normal C# code in Visual Studio, you have to add "using" statements in your editor document for the namespaces so that you can directly use types within it. It should be pretty straigtforward though, as long as you followed the "Getting Started" steps and then included the proper assembly references.
Let us know if you have further questions.