Ok, so the Button is used for several things ...
First : one rapid click on it : Your Nabaztag will say again all the messages he received and which are not yet archived.
then : Two rapid clicks : the rabbit will archived all the messages.
if you click on it during he says a message, it will stop it.
and for ending : if you push the button for a long time, you will be able to speak with your rabbit asking him the weather, the market and so on ! But it's not available yet for all the rabbit : next week !