what does your settings for the plug array look like?
it should look similar to the following:
$rcmail_config['plugins'] = array('settings','hmail_autoresponder','html_password')
In the pluginname/config folder, do you have
config.inc.php
config.inc.php.dist
have you configured the DSN in the setting in config.inc correctly?
$rcmail_config['db_hmail_password_dsn'] = 'mysql://login:password@localhost/database?new_link=true';