I'm on the Chrome Developer Relations team (and an avid user of ABP). I'm upstreaming a bug we just dealt with on our tracker.
It appears that while ABP is enabled, the New Style Rule within Chrome DevTools doesnt work. This the + icon in the styles pane.
Details and repro are within here: http://code.google.com/p/chromium/issue ... ?id=138854
Would love to see what we can do to help address this.