Download the latest atrpms-repo rpm from http://dl.atrpms.net/el6-x86_64/atrpms/stable/
Install atrpms-repo rpm:
rpm -Uvh atrpms-repo*rpm
Install bonnie++ rpm package:
yum install bonnie++
Source: http://pkgs.org/centos-6-rhel-6/atrpms-x86_64/bonnie++-1.03e-6.el6.x86_64.rpm.html#howto