If you are getting this error about credentials, you are reaching the database correctly. Where did you get the password for the user bn_liferay? You have it in the portal-ext.properties file. Apart from that, the admin credentials are 'root' and the password you have used during the installation.
Besides, this MySQL Workbench is in the same machine as the Liferay application? If not, you cannot use the user bn_liferay@localhost, because it is trying to access from a different machine and it is not at (@) localhost.