Adapt Dynamixel library to different shields
You are setting the baudrate to 57.6K however, you need to communicate with the Dynamixel at the baudrate it is set to. I believe the Dynamixel comes with a preset baudrate of 1Mbs. The first thing thing I would do is to change your 57.6K to 1Mbs and try that. This is assuming the ID is actually 1 (also a default) 

Here are two links for info on Arduino baud rates and Dynamixel serial communications.

Controlling AX-12 Servos

How high of a baud rate can I go (without errors)?