The flat side of the base is for guiding against a straight edge. However, you must use the straight based on the direction of feed so that the rotation of the bit forces the router against the guide, otherwise you'll be fighting the router while making the cut.
Take some scrap and use the edge guide, and check the rotation of the bit. It will work in only one direction, try pushing the router away from you as you make the cut then try pulling the router towards you making a cut you'll see a difference.
hope this helps.. Woodnut65