firstly - I know this is obvious but are you using the correct password for the phpmyadmin interface for the correct database? Always worth checking.
It says you don't have the mcrypt extension installed. If you are using ubuntu (maybe with debian):
sudo apt-get install php5-mcrypt(assuming you are using php5... and you may need todo a apt-get update first.)
Let me know if this is of use... I have some manual ideas too.
More info on mcrypt can be found at:
http://uk.php.net/manual/en/mcrypt.requirements.php - its better for phpmyadmin but not required, and you can turn of this need, but I'd recommend try to install mcrypt. Let me know if you haven't got ubuntu (or debian). To disable the need you need to edit config.inc.php setting:
$cfg['McryptDisableWarning'] TRUEThis then disables the default warning that is displayed if mcrypt is missing for cookie authentication. You can set this parameter to TRUE to stop this message from appearing.