Oh dear (this new inet kiddy mentality really drives me ).
If you are interested in WhatsApp functionality on N9 please consider to contribute! At least a bit of time for searching in the proper thread. Instead of just spitting out a question and let the others do the work.

Spoon feeding:
beginning a conversation seems to have stopped a few months ago when WhatsApp again changed their protocol and no N9 dev around to follow. Syncing contacts is not possible anymore. So you have to ask them to message you and then you can add and answer them. Or you do edit the conf database on your own (will need more spoon feeding, I guess )

P.S.: feel free to ask any mod to move these two posts