I had my web host install wordpress for me. In the process it created a MySQL database, and generated for it a username, pw, and database name. When I checked the wp-config.php file via a PHP client, I noticed it had automatically copied the database username, pw, and database name, so I let the wp-config file alone, not making any adjustments.
And that is all you needed to do :-) There is no need to create another db user, etc.
When you say that the web host install WP, did they actually do it or did you use their "automated installer" (QuickInstall, Softilicious, Fantastico, etc.)?