Since the hardware is connected through Ethernet which uses the Ethernet protocol (Not RS-232) it will not appear as one of your serial devices on your PC. You can use the NI-VISA server on your hardware (sbRIO) to be able to communicate it using VISA. To do this:
1) Right-click the Software tab
under your hardware in NI-MAX then choose Add/Remove Software
2) Choose the compactRIO driver you are using.
3) Mark NI-VISA Server as one of the pieces of software to be added.