It should already be marked as answered, since you selected answers as a "good answer".
We just deployed a new script today that emails people after one week if their question is still "unanswered", and there may be a bug there. We'll take a look. Thanks for letting us know!
Edit: I looked at timestamps, and I think we emailed you, then you came and marked several responses a "good answer", which marked the question as answered, and removed the "mark as answered" button the emails talked about.