well, technically you cant, since meego, the software that powers N9, doesnt support whatsapp, however there are to ways to get around the problem
first download Wazzap, Wazzapp is an unofficial port of WhatsApp
and quickly became one of the most downloaded apps for Nokia N9.The app allows you to communicate with your buddies who use WhatsApp on their mobile phones.
you can get it from http://openwhatsapp.org/download/
second alternative is to get a geek like me to load Android icecrea fro you on the N9 and it becomes an android device...