In brief, you have two options to report your issues:
(1) Post topublic-dns-discuss forum. This shares your report publicly, and allows you to get feedback from other Google Public DNS users.
(2) Post to theIssue Tracker. This keeps your report private to yourself and Google Public DNS engineers. Your issue will be given a tracking number, and a Google Public DNS engineer will be assigned to handle it. You can view the issue's history and status as it is updated.