AutoMapper is used to map the ModelsBuilder Api to Umbraco ViewModels. Dave Woestenborghs has written an excellent article on 24 Days that you can read for more information. This step-by-step is basically a paired down version of his article and a heads up for gotchas we encountered when setting up our project.
(If this isn't your first project using this method, you already have this installed, so don't worry about it!)
Download and Install Stephan's custom ModelsBuilder tool for VS: