If you're running cpanel on your site, the link to phpmyadmin would be something like
http://www.yoursite.com:2082/3rdpart...dmin/index.php
If it's another hosting panel thing, it's likely to be in a "MySQL Databases" section.
When you're in phpmyadmin, select your forums database from the dropdown box, click the SQL tab, and upload the sql file in the "Location of the textfile:" box.