In SiteDelta Watch, click the expand button on the left side, below the yellow information bar. There, you can add scan and ignore regions by clicking "add new" and clicking in the corresponding area in page shown on the right (you may have to move the mouse around until you find a place where the right region is selected as shwon by the outline).

Advanced users can click the "add new" button twice and manually enter the xpath selector to define the region.