Short answer: MySQL Server is the same as the hostname you added to the database on the Goodies -> MySQL panel. Did you add a hostname?
The MySQL Server is the address of the computer that is hosting the MySQL database. For example, sites might have an "FTP site" for downloading files such as ftp.example.com, well with DreamHost you have an "MySQL site" for each database.
So first things first and thats to login to the DreamHost Web Admin Panel. Go to "Goodies -> MySQL" and click on "Add New Database" link. It will ask you to choose a name for the database. Then it will ask you what plan the database will be on.
Now you have a database allocated on the MySQL server. You then have two more things to do:
1. Set up a user
2. Set up a hostname
1 is self-explanatory, its basically the credentials used for a database connection and the permissions granted.
For #2, you need to choose a hostname that would be a subdomain of one of your hosted domains (ie example.com -> mysql-hostname.example.com) but since you haven't updated DNS yet try seeing if you can do mysql-hostname.dreamhost.com instead. This hostname must not be a currently active hostname, and you can't use it for other services! This means you can not use www.example.com for both HTTP and MySQL for example.
It will take sometime for the database to be ready. The Web Panel says about an hour. You'll know its ready when you can browse to the hostname in your web browser and it asks you to enter the username and password. If you login as the user you added to this database, you can use phpMyAdmin to manage the database.
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