IT WORKS !
In fact, the pkg file did not instal all the requested files : Stats.pm was not installed at all and local.cf was not updated with the spamstats SQL infos.
I have just read the readme file included in the pkg file and it was the clear for me !
Moreover, the readme file says that one must do some "changes" for the spamassassin "server" AND for the spamassassin ispconfig. as i have no spamassassin "server" following the perfect server setup, it has mistaken me for some times ...
But it's clearer now !
... and the mysql database was NOT installed at all ! I've done it by hand !
Do some other people had the same issue ???