If your account has a YouTube preroll ad included, you can access it the same way that you access your video montage. From your dashboard's overview page, click on the red Manage my Video tile.
You will be directed to a new page with two panels: one with the status of your video montage and one with the status of your YouTube preroll ad.
Click on the blue button to Manage this YouTube Pre-Roll. From there, you will be directed to a new page that shows either your YouTube ad’s script, status, review panel, or your approved ad with embed codes and other important links, depending on where you are in the production process.