Do you have Kobo Desktop installed on your mac? If so you will find all the kepub versions of your ebooks in there.
I don't have a mac so can't help you with the location of the Kobo Desktop files, but, as far as I can see you don't need to know the location if you use obok in Calibre to convert them to epub files.
obok will locate the Kobo Desktop files and they will appear in the obok drop down box. Choose the ebook/s you want to change the format of and they will be imported into Calibre. That's all you need to do.
You will only need to download Adobe Digital Reader if you want to download the Kobo ebook as an epub file. Irrespective it will auto download to a kepub version on purchase, and you will find it in Kobo Desktop.
Let us know how yo go.
