You have figured out how to report the spam from your Inbox but your system uses Postini to hold most of the spam getting to you and you would like to report it as well.
I do not report the spam that is caught by Postini. I don't think there is an easy way to report directly from Postini because you can't forward messages to an alternate address. If you have filtering capabilities on your machine, you can select and "Deliver" to your own address, and make sure to filter to an alternate folder (X-pstn-disposition: quarantine should work). Then report as normal.
If reporting spam that has slipped by the Postini filtering, you can also include spam[at]postini.com when submitting via email so they can adjust their filters.