I’m using Google Ad Manager because OpenX didn’t get back to me quickly enough when I requested a free hosted account. Sorry, it’s not Google Ad Manager, it’s Double Click for Publishers. Whoops.
It’s relatively complicated software, probably because serving ads is relatively complicated business. I have hacked my way through it, but the single most helpful tip I found was to append <code>?google_debug</code> to the page you’re trying to get the ad displaying on. This pops up a window with what is basically a stack trace of the javascript execution on your page. You might not understand all of it, but you can definitely cut and paste the message you find into the Google Ad Manager help search, and find more useful answers.