There is no official WhatsApp iPad app, its frustrating. What's more frustrating are the apps in the app store which allow you to access the WhatsApp website while looking at adverts and dealing with pretty bad lag. Here is a method to get WhatsApp working on iPad by using Google's Chrome browser.
1. Load up Google Chrome and go to https://web.whatsapp.com/ , it will take you to their homepage
2. Click on the Chrome menu and choose "Request Desktop Site"
3. Scan the QR code with WhatsApp on your phone, enjoy!