ISPmanager isn't used to update software. Install PhpMyAdmin 3.x manually downloading it from a site of developers (
http://phpmyadmin.net) or find CentOS documentation or visit forums of a site
http://centos.org to get the appropriate information on how to install this version of PhpMyAdmin from repository on CentOS 6.0.
Anyway, Installation and setup should be made manually either from archive or from a repository