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Kobo reader: how to open the root folder

I’m trying to install Koreader, the alternative PDF-reader on my Kobo Aura One. Installation instructions say I have to install it in the root directory. When I connect the Kobo via USB to my Mac, I see a folder with all the books. Copying to that folder doesn’t result in installation it seems.

How can I copy files to the root of a Kobo reader?