On PC, you can't use the Web version of WhatsApp since there is no dedicated client. For this to work, the phone needs permanent Internet connection and the computer you're using. Once everything is met, open WhatsApp and press the three point menu then select WhatsApp Web. On your computer, go to http://web.whatsapp.com and use the phone to scan the QR code. After this procedure, you will be able to use WhatsApp with the phone nearby.
