Some users are currently seeing a '-' in the earnings column of the Monetize tab, even if they have generated earnings. Please be assured that this is only a display issue, and that your earnings are being recorded.
We are working on a fix, but in the meantime you can view your earnings by logging in to your AdSense account directly at http://www.google.com/adsense
Update, 2/17: This is fixed. Thanks for your patience.