I am using MAMP and I changed my web server folder to a folder on my desktop. I downloaded the latest version of phpMyAdmin and placed it in the folder. When I navigated to it using localhost:8888/phpMyAdmin I am logged in automatically. I created a config.inc.php file from the sample but there is no option to add a username or password.
You should refer to the documentation, in particular it explains that phpMyAdmin doesn't have user accounts. The user accounts you use to authenticate actually are MySQL accounts, so to add a new user you'll have to add a MySQL user. You can do that from the phpMyAdmin "User accounts" tab (called "Privileges" in older versions).