After installing amavisd you need to edit the confiuration file to change some of the settings:
# vi /usr/local/etc/amavisd.conf
:
$mydomain = 'yourdomain.com';
:
$final_spam_destiny = D_PASS;
You also need to enable clamav section, then restart:
# /usr/local/etc/rc.d/amavisd.sh restart

To allow amavisd-new to work with clamav you need to run clamav using the same user:
# vi /usr/local/etc/clamav.conf
... change user to:
User vscan
# /usr/local/etc/rc.d/clamav-clamd.sh restart