squirrelmail no such user `www-data:www-data'

I want to configure squirrelmail on ISPConfig 2. I have Mandriva Linux 2009.1 . I am configuring using http://www.howtoforge.org/squirrelmail_ispconfig . In step 6 I am writing :

chown -R www-data:www-data data
chown -R www-data:www-data sqmattachments

but I receive an error

no such user `www-data:www-data'

 

Thanks it worked. On mandriva use and group are apache.