Sometimes it could be difficult to get a proper connection established between the CMe3100 and your PC, when using the USB cable. 
Here are some tips that could make it work.

1. Make sure that you have the latest version of the CMe3100 driver installed on your computer.

you find the latest version of the setup file here:
https://support.elvaco.com/hc/en-us/articles/115005880865-CMe3100-setup-file

2. Make sure your computer has been rebooted after the you have installed the software mentioned above.

3a. Make sure you are using the correct USB cable and the correct USB port on the CMe3100. It should be connected to the USB-mini on the bottom of the unit.

3b. make sure there is no network cable connected between the unit and your computer.

4. Make sure the CMe3100 appears as an USB-device on your computer. if not, try another comport and/or reboot the CMe3100.

5. When the CMe3100 is connected, the "Elvaco Serial Port" should be visible in the device manager under comports. If not, the driver has not been properly installed, allowed or recognized by your computer system.

6a. Start the device with the "configure CMe3100" shortcut located on your desktop.

6b. try to connect to the device via 169.254.254.1 in your browser.

6c. If the device still doesn't appear on your PC after this it could be worth trying a factory reset.

You perform a factory reset by powering off the CMe3100. Press the button located above the"M-Bus master connection" and power on the unit and release when both the Green and Red LED flashes. Wait some minutes, and reconnect the USB cable. Try to connect with your "configure CMe3100" shortcut on your desktop.

7. If this still doesn't work, try to enter the device by using IP 169.254.254.1 in your web browser.

8. If this still doesn't work, unplug the Network cable from the CMe3100, and disconnect from all networks that your PC is connected to. Disconnect the USB cable again, reconnect it, and try to reach it via the "configure CMe3100" or the IP 169.254.254.1 again.

If you have done all these steps and it still doesn't work, something on your computer is refusing the connection to happen, or the CMe3100 needs to be replaced. Sometimes it could also start to work if you wait a bit or disconnect/reconnect the USB cable several times. To try another computer could be a way forward.
If the device properly appears as a removable disk on your PC, enter it and open the machine.info file with notepad. Look at which software the device have. If it uses 1.9.2, you will not be able to connect to it with the USB. Then you need to return it to Elvaco for a replacement.

Note: Much older CMe3100 devices uses another IP (192.168.100.1)

also note that if you manually change the port in the web interface, you must specify this when you try to connect to the IP, eg 123.123.123.132:8080