Available in 3.0:

For the most basic visualization, all you need to do is call the Model method setUseVisualizer() prior to calling initSystem():

    Model myModel;
myModel.setUseVisualizer(true);
// usual stuff ...
myModel.initSystem(); 

This should result in a graphics window popping up with a reasonable display of your model in it.

For more information, look in the OpenSim API's Doxygen documentation for the ModelVisualizer class. For advanced users, OpenSim::ModelVisualizer is based on the SimTK::Visualizer class that you can read more about in Simbody's Doxygen documentation.

More to come ...