The WPS button is on the router. It is either a real (hardware) button or a software button triggered from the router application.
See this; it is pretty generic.
http://usa.canon.com/app/html/web_tutorial/WiFi-MX870_350/
John Hoffman
Conway, NH
1D X Mark III, Many lenses, Pixma PRO-100, Pixma TR8620a, LR Classic
