CHMOD your modules/Forums directory 777

then re-uplod the install.php file from the pack
html/modules/Forums/install.php
if it is no longer in your Forums folder on your FTP. Delete the config.php file in your modules/Forums if there is one there and re-do the install. Click on Forums, it should ask you to build the database again.

Basically it sounds like the config.php file did not get created or created correctly for the Forums when you first accessed the Forums to set them up.

mikem