A long time ago I did this. From memory if you have registration goals established already, then you should be able to simply access that data to see where a click comes from. The problem with ads is that a user will click them through to your site, then watch several pages, maybe go away, come back, then register. That is where tracking becomes lost for such purpose.
You may have to setup a funnel specifically for anyone who found your site from an ad in the past 180 days or so, regardless of further browsing history, then register. I find many a user who has posted that they came to the site from x... read for months... then registered.