I always used the wazzap method and it worked every single time (i flash my phone frequently due to being a newbie in hacking).

so be sure to follow the order exactly

1.install wazzap
2.install wazzap path (i never reboot after that as it says)
3.open wazzap and register your number
4.the code should arrive by sms and inputted automatically.
(if that doesn't happen try the voice call option from wazzap.for me worked every time i got no sms)
5.after the code arrives(which it should)install cepi's latest whatsup from warehouse.
6.open it and you will see an existing wazzap account is available.
7.import it , close the app , uninstall wazzap and have fun with the wonderuful WHATSUP.

Hope it helps and if it doesn't then i hope that someone who really knows about those things helps you.
CHEERS!