Launch Safari in a regular tab (not a Private tab) and go to http://play.geforcenow.com. You will be prompted to create a home- screen shortcut that you will use to launch the GeForce NOW app. If your home screen icon is not the GeForce NOW icon, you may have launched http://play.geforcenow.com in Safari Private mode. To fix this, please delete the GeForce NOW home screen shortcut and recreate the home screen shortcut with Safari Private mode disabled.
For EU users, additional instructions here.