In case you had WhatsApp or WhatsApp plus working and it stopped working recently,
1) Take a WhatsApp Chat backup, if possible on your device ( otherwise it will anyways be there if WhatsApp was working recently)
2) Uninstall the WhatsApp or WhatsApp plus on your device
3) Install the latest WhatsApp Plus from here
In case you are not able to register to WhatsApp at all,
2) Clear app data (go to settings > apps > WhatsApp > clear data)
3) Change date to 25th Jul 2014 or (even before if it won’t work) (go to settings > Date and time > uncheck “automatic update of time”),
4) Reboot the phone and you should be able to register and use WhatsApp. Only that your messages will be backdated.
5) Uninstall Whatsapp now
6) Set time to present date
7) Install the latest WhatsApp Plus from here
Do let us know if it works for you.