I suggest you check out the behaviour of the motor controller using plain old Telnet and make sure you have got the communication protocol, addresses and ports correct. This will also enable you to confirm that your use of Wireshark actually does show the network traffic you're sending. Be sure to run Wireshark on a separate machine from your Telnet client, by the way, to prove that it is successfully capturing traffic on the interface in promiscuous mode.
Once you have that working, connect your Arduino to the same LAN port as the machine that you were running your Telnet client on, make sure your sketch is using the same TCP/Ip address and port, and sending the same string as you used in testing. If all our assumptions are correct, you should see the same network traffic, and the motor controller should respond the same.