If you are routing a decorative edge on the surface of your project, the attachment will work best with a piloted router bit like the #615.  The pilot on the bit (a pilot is a guide section of bearing on the end of the router bit designed to follow the edge of the wood) works to follow the exact edge of the wood while the cutting is taking place.  Piloted bits will not work down the center of the material to rout a channel.  The pilot will burn or break off.  If routing a decorative edge, use the attachment without the edge guide.  If you are routing a straight channel or line 2 - 3 inches from the edge of the work piece, use the edge guide and a straight router bit like the #650, #652 or #654.  The edge guide will keep your line a consistent distance from the edge.