Ok, I've found that. It is not mentioned in the manual.
I am able to use EthernetSettings to set the IP address, listening port and set to "Enabled". (how do you disable it again??)
However, when I go to PLW Recorder, and "settings", then select CM3(ethernet), there is no device to select. (unlike when I select CM3(USB), where I can select a device)
Even if I put in the IP address and listening port down the bottom and hit refresh, there is still no device to select.
Disable it by setting the IP address to 0, although there is no issue with leaving it in and just unplugging the device.
What IP address and port have you selected ?
Can you ping the CM3 from a command prompt window on your computer ?
Do you have a firewall enabled on your PC blocking the telnet requests ?
Is the CM3 on the same subnet as your PC ?
The device will automatically detect powe over ethernet if the switch it is connected to provides this, no configuration needed.