Menu -> settings -> import and export -> import mail -> import generic mbox -> add folder".
Point it to that account folder (or the specific year folder under that account folder).
Then, choose to import into a new account or an existing POP account.
That's the quick fix for it. Sounds like you have some database problems etc., but if you just want to see the messages, that should do it.
Actually the line may already be there - but without a path after the equals sign.
There's a note about that in http://www.reddit.com/r/operabrowser/wiki/m2tips/enable_logging.
This added line is automatically deleted when starting Opera mail
If you indeed put the line under the section for the account in question and you're editing the file while Opera is closed, it might have something to do with the path you're using. Maybe try a path that's under C: if you haven't yet. Specifically, try a path on your in your desktop like in the above link. Another thing you can try is to set the incoming log too as maybe that will prevent Opera from erasing the changes on you.