It works just like other services in the server version. Yet, how difficult to also leave it accessible in System Preferences as in the standard version? Only the developers know. The standard version is simplicity itself.

For the server, a couple of steps. Just like all other services, it must be enabled. Server Admin-><server in question>->Settings->Services->Check the MySQL box.

http://www.apple.com/server/macosx/resources/documentation.html

That page is a listing of PDF guides to aid in the administration of a Snow Leopard server. "Web Technologies Administration" is the download that covers MySQL for Snow Leopard Server, OS X version 10.6. Many neat things, like starting and stopping various MySQL versions is covered in the Apple docs