Follow the [phpmyadmin set-up directions|http://www.phpmyadmin.net/documentation/Documentation.html#quick_ins tall], really. It's basically the same sequence.
As for your questions, the settings are in your phpmyadmin directory, in a file usually named config.inc.php, and there's usually also a default version of that file named config.default.php in that same directory.
You can edit the configuration file (launch Terminal.app, then
cd to your particular local phpmyadmin directory, and +nano config.inc.php+ to edit the configuration file, and look for the username and password in there. The lines are usually something like this:
$cfg['Servers'][$i]['user'] = ...
$cfg['Servers'][$i]['password'] = ...