First check Phpmyadmin works or not, then see if you can create databases.This will confirm you have done everything correctly or incorrectly.
If you have installed it using fantastico or softaculous then find db user name and db name by clicking them in cPanel.
If you have installed it manually then find user name inside configuration files (If you don't remember db user name). And if no users exist in cPanel's mysql section then create one and allow it to use the database that your forum uses.This user name and its password should be the same as it was at the time of installation.
Hope this helps