WordPress MU

Hi,
I can't get MU Wordpress running
I have the same error that some of the users have
But I can not find the right answer in the past threads about this problem, would you plz help me to fix it ?
this is what I get after installation :
Fatal Error
Database Tables Missing.
The table username_mupress::wp_site is missing. Delete the .htaccess file and run the installer again!
Please check that your database contains the following tables:
wp_blogs
wp_users
wp_usermeta
wp_site
wp_sitemeta
wp_sitecategories