If this actually works for the competitors I want to track, this is an awesome product!
YellowBrix, for example, is currently running search ads, but not on Google Display Network. We're confident that we're finding practically every advertiser and AdSense text ads right now, so if you're not able to find any of your competitors in our index then they either haven't tried or weren't successful at it.
1. Search some of your main keywords on Google.
2. Look through the organic results and find organic results pages running AdSense.
3. Search the display URLs of advertisers placing AdSense ads on those sites in MixRank.