If you have installed the plugin but can't see the icon in your browser, firstly please try restarting your browser. If the icon still doesn't appear please check:
In Firefox: Settings icon > Add-ons > Couponize > Options
In Chrome: Settings > Extensions > Options
..and make sure the 'Enable icon' checkbox is checked.
NB: check that you are using the desktop version of either Firefox, Chrome or Safari - Couponize doesn't work with mobile/tablet versions or Internet Explorer, currently.