If you have Installatron on your VPS, it is incompatible with Zend 3.3.0. The Installatron links will not show up properly in your control panel with both installed. You will need to stick to version 3.2.8 which works fine.
I believe that 3.2.8 is still the default version for DA and customapache.
To revert, you can follow the directions above for installing Zend. This should revert you back to 3.2.8
FYI I did not need to manually install Zend Optimizer on my new KnownHost VPS account. It came with PHP 4.4.9 with Zend enabled.
However, when I upgraded to PHP5 using DirectAdmin's custombuild tool, I did need to specifically enable Zend in the options.conf file. It was disabled by default. Instructions for using custombuild to upgrade to PHP5 are here:
[Because I'm a new user, the forum helpfully won't allow me to post clickable links. You will need to copy and paste the link above.]