If you drop an epub to the root of the kobo, instead of into subfolders, it should show up just fine. (So long as it's not DRM-ridden or very poorly coded.) Alternatively, Calibre is pretty simple unless you want to do something complicated, in which case it has just about every feature you could possibly imagine. Just use the setup wizard instead of playing with the settings yourself.
All of this info and a lot more is on these forums, which many people find more useful than Kobo's official help - which is, unsurprisingly, geared toward people who want to simply sync official Kobo books rather than toward us tweakers and sideloaders. Start with the sticky threads in the Kobo subforum - FAQ and Tips & Tricks. The first post will point you to the Wiki, which has this subthread:
https://wiki.mobileread.com/wiki/Kobo...obo_eReader.3F