Hey Nick,
This form is merely for bringing an unattached database under control of a virtual server–it does not import a database into MySQL. The database must already exist in MySQL for this to form to do anything with it. There is a way to import databases into MySQL from Webmin, though. But there are some caveats.
It isn’t safe to copy a raw MySQL database from one system to another, unless they are running [em]identical[/em] versions of MySQL (and I mean exactly the same–not even a minor version difference). If there is any difference you need to do a dump on the other system–the MySQL module can help you with that as well (you can even connect to it remotely, if you don’t have Webmin installed on the other box…but MySQL would have to be listening on a public IP and port).
Anyway, to transfer a database from one system to another assuming you have Webmin on both, here’s how you do it:
On the source server, browse to the MySQL Database module, and click on the database you want to migrate.
Click "Backup Database".
Fill in the form–generally you just need to give it a filename. All of the other stuff makes the backup selective, compress it, schedule periodic backups, etc. Click "Save and backup now".
When the dump finishes, copy the file you just created over to the destination server.
Now browse to Webmin’s MySQL module on the destination machine, and click on “Create a new database”. Give it a name and create an empty database.
Now click on the new database in the list of databases, and click "Execute SQL".
In that form, you can select the file you copied over from the other server.
Done. After this, you can then import the database under control of a Virtual Server in Virtualmin.