To connect your device to your computer, use a USB A <> USB B cable.
Insert the square connector in the device, and the USB connector in your computer.
If the proper software and driver are installed, the scale will now communicate with your computer.
Check this article to read what cable you need to connect the MC-580(M) with your computer.
Check this article to read what to do if the MC-780MA doesn't communicate with your computer when the right cable and software are used.