Hi,
There should be a folder called install, you'll have to run that.
http://yoursite.com/forums/install
It will give you instructions on what files you need to chmod. To chmod a file, select it in FTP, and select the numbers listed in the installer.
It will also ask you info on your database. You'll need to setup a new database and user in your hosting control panel.
If you need any more help just ask,
Nathan