I just posted a similar example here:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1278919947
Use the interface circuit from the AX12 manual using HC126. You don't need the inverter, use the outputs on D2 and D3. You would only need the USB2Dynamixel to talk direct from PC over USB to Dynamixel.
If you want an arduino based board for controlling dynamixel, then Arbotix is best because it has two UARTS to talk to servo at the same time as the normal serial . Also has interface and library already.
For specific dynamixel help, try trossen or robosavvy forums.