Are these books that you have downloaded using the Kobo Desktop app or by using My Books menu on the Kobo website to download?
If they were downloaded using the Kobo Desktop app, you may want to look into the Obok plugin which is part of the DeDRM package. To use this, download DeDRM_tools_10.0.2.zip from
https://github.com/noDRM/DeDRM_tools. Open the .zip file you downloaded and extract the DeDRM_plugin.zip and Obok_plugin.zip files. In calibre's Preference, open Plugins and then click on Load plugin from file. Browse to the Obok_plugin.zip and select it, click yes on the security risk popup.
If they were downloaded using the My Books menu and they were DRM free, you can look for them as files with names that look like a GUID with the epub extension. If they were Adobe DRM files, you will need to use ADE authorized with your Adobe ID to download them. Alternatively, if you already have ADE installed, you can add the DeACSM plugin to calibre which will allow you to open the .acsm files without needing ADE. In either case, you will need the DeDRM plugin installed.