Possible causes of friendly URLs not working (usually manifests as "404 not found" error on forum pages):
1) Your server doesn't have mod_rewrite installed.
2) Your server doesn't have AllowOverride enabled:
http://httpd.apache.org/docs/2.0/mod/core.html#allowoverride
3) The .htaccess file is not in place in your forum directory.
4) Sometimes you need to specify RewriteBase in your .htaccess file (specify the name of your forum directory):