Did you follow all of these steps?
After install you need to add your root mySQL password found
in /root/.my.cnf file to:
/opt/administration/phpmyadmin/config.inc.php
$cfgServers[1]['password'] = 'yourmysqlpasswordhere';
Then access via:
https://yourdomain.com/phpmyadminlogin with the admin username/password
(dont forget you must use https instead of http)