Login to WhatsApp requires an active WhatsApp account on your phone. To scan the QR code you need to open WhatsApp application on your phone.
- On iPhone: in WhatsApp application go to Settings > WhatsApp Web.
(Not Setting on your home screen, but the small gear icon within Whatsapp)

Note: You need to be running iOS 8.1 or higher to activate WhatsApp web. iPhone 4 doesn't support iOS 8.
- On Android: in the Chats screen > Menu > WhatsApp Web.
- On BlackBerry: go to Chats > Menu > WhatsApp Web.
- On BlackBerry 10: Swipe down from top of the screen > WhatsApp Web.
- On Nokia and Windows Phone: go to Menu > WhatsApp Web.
Read more details at WhatsApp Support page.

From your phone, navigate to WhatsApp Web to view your logged in computers or to logout from an active WhatsApp Web session.