Hello Alan, it's hard to say what might be wrong without seeing you do it. Possibly giving us a quick call might be the best way for us to help you.
I would check that you don't already have a route plotted; if you do, the touch-to-add-waypoints thing will be disabled. If you want to extend an existing route, first touch the last waypoint at which point you'll get a speech bubble encouraging you to touch the next one. If touching the next one doesn't do anything, something is wrong. Ensure there actually is a waypoint where you're touching. But I think giving us a call would be very helpful.