Ea is right, you should see the Kindle pop up as a hard drive when you connect the USB-cable between your computer and the Kindle. When you click on the hard drive for the Kindle you should be able to see the documents folder. All books is supposed to go in the documents folder.
I find it easier to transfer books directly rather than transfer them through Calibre.
Here's a more thorough explanation on how to transfer files to the Kindle:
https://www.amazon.com/gp/help/custo...200375630&#usb