Just to verify, place the Boundary Layer as the top Layer. From the Layers tab, go to the view box and make sure the Field Boundaries is Highlighted. From the Red Barn tab, highlight the field name you added the extra acres for. If you still see the line then right click on the same field name and select Edit Boundaries. Then select both sections of the polygons, right click and select Merge. If this does not function, then use the Include polygon tool and trace around the line splitting the map, right click and finish. Click Save and Close.