It's been renamed:
var viewportAdapter = new BoxingViewportAdapter(Window, GraphicsDevice, 800, 480);
_camera = new OrthographicCamera(viewportAdapter);
Once you have the camera you can use it to convert from screen to world and visa-versa.
_worldPosition = _camera.ScreenToWorld(new Vector2(mouseState.X, mouseState.Y));
Take a look at the demos in the github repository for more examples like this.