ThatsBoBo - 3:24 pm on Apr 4, 2012 (gmt 0)
The feature should be live for all accounts by now. Are you using IE8+ or Chrome, etc? IE7 or earlier will not display the feature.
It can be found under the Allow & Block Ads tab -> Ad Review Center -> Shown Ads.
The feature was announced on March 22nd and was to be rolled out to everyone within "a few days"... I've had it since the 23rd.