Well, if the downloads can’t be completed, of course they won’t extract. So it’s not an unarchiver issue, it’s a download issue.

Can you give us more specifics on the problems with jdownloader? Are you hitting Mega’s daily 5gb limit for free users?

I don’t use Mega often and don’t have a Mac anymore, but the more information you can provide the better we can help you. Just saying “It’s not working” isn’t going to get us anywhere, as there’s about a million reasons it could be failing. More info helps narrow it down.

Which parts have completed downloading? Which parts haven’t and what kind of message are you being given about the parts that are incomplete? Are they showing as still downloading, are they showing as completed but corrupted, are they showing as stalled…?