To use postfixadmin you need a web server, with PHP support enabled. It's time to check if the httpd package is installed, along with php. CentOS default php package does support MySQL, so it doesn't require modifications.
If the packages are missing, then install them:
Postfix scripts are available to download at http://sourceforge.net/projects/postfixadmin/files/. Download get the .tar.gz file and extract the files from the tarball into a directory, preferably below /var/www/.
Now, create the database which will store information about domains, users, passwords, etc: