The first issue has to do with your user table. There should be a column in it called 'warnings'. Add it by running the following query:
ALTER TABLE " . TABLE_PREFIX . "user ADD COLUMN warnings int(5) default '0'
As for your 404 error, that means that the system can't find the Warn.php file. Make sure that the file exists in your forum's root directory (where the directory admincp is), in other words in :
http://www.websitename.net/forums/
Rgds