That section of the phpMyAdmin config file should look something like this. Make sure the user that you specify has the proper privileges and that the hostname matches what you specified for that user.
$cfgServers[$i]['host'] = 'hostname.host.com'; // MySQL hostname
$cfgServers[$i]['port'] = ''; // MySQL port - leave blank for default port
$cfgServers[$i]['socket'] = ''; // Path to the socket - leave blank for default socket
$cfgServers[$i]['connect_type'] = 'tcp'; // How to connect to MySQL server ('tcp' or 'socket')
$cfgServers[$i]['controluser'] = ''; // MySQL control user settings
// (this user must have read-only
$cfgServers[$i]['controlpass'] = ''; // access to the "mysql/user"
// and "mysql/db" tables)
$cfgServers[$i]['auth_type'] = 'config'; // Authentication method (config, http or cookie based)?
$cfgServers[$i]['user'] = 'mysql_username'; // MySQL user
$cfgServers[$i]['password'] = 'mysql_password';
If you added a user for phpMyAdmin using GRANT, don't forget to use FLUSH PRIVILEGES.