It's an .htaccess problem.
Did you upload the included .htaccess file to the directory? That is what forces the "forum" file into php mode. If you're running apache, should be no problems. If you have uploaded the .htaccess file and it's still not working, then try uploading the .htaccess file into your root directory (lowest directory you can go to) being careful not to overwrite any existing .htaccess file. You may just want to append it to the exisiting one.