I think you only had to enable a mySQL database for that website via the basic tab of the site in ISPConfig.
After that, you could click on the options tab and define a database for Joomla.
Via PHPmyAdmin you could drop the MySQL dump (provided by Joomla) into the field for that website.
(Maybe you've done it, but you're not specific at this point); to enter ispconfig do not point only to http://yourIPaddress/
but to http://yourIPaddress:81/