$rcmail_config['plugins'] = array(sievereules);
should be
$rcmail_config['plugins'] = array('sievereules');
and in the sieverules folder you'll find a file called config.inc.php.dist this should be renamed config.inc.php and you need to go through this file, filling in the config variables to the values you want, like the address of your sieve server. there are comments in the file that explain what the variables are, similar to the main rc config file.