Sorry, i couldn't right sooner...
In fact, i just wanted to be able connect as an "admin" in horde to access the administration pannel.
I installed successfully the nag addon (directly from horde, the tgz one). I created the mysql database. I had to setup nag in horde and THIS was my problem. so i did this :
I edited the /home/https/html/horde/config/conf.php and i added a user there :
// 120Authentication
$conf['auth']['admins'] = array(admin@localhost);
I now it's juste temporarly but it was enough to be able to setup nag...
Now, it works great....
Michaël