Error “Zend Optimizer not installed”

I installed a theme from some website and when I tried to activate it, it start giving me an error which reads “Zend Optimizer not installed”. I tried installing it on my local machine but that’s of no use as it need to be installed on my web server. I am not able to navigate any page as it gives me the same error page and ask me to download the software using http://www.zend.com.