Hello World,
have sombody experience with the Servo SR518 (Dynamixel compatible)
with 4 wire:
-GND
-VCC
-A (RS485) half dublex
-B (RS485)
I found some Information from Servo Type AX (it has 3 wire) but nothing about SR518 (Spinr RC, China)
It looks like a typical RS485 connection.
Try http://arduino-info.wikispaces.com/RS485-Brick to get a jumpstart on using RS485. Beyond the wiring and how-to-send-messages part it looks like you're going to need to do a lot of reading on the Robotis site to learn the proper messages to send.
May be this will help
It shows some videos on controlling the SR518 servo using Arduino and also processing.