You receive this message if you attempt to connect to MySQL without telling our system which actual MySQL process you wish to connect to.
When you created the MySQL process, you selected a DSN (also called its hostname), which is the name you must use to access your process. This name is of the form something.db and is also identified in your MySQL setup email and on your MySQL panel.
Most software packages that use MySQL offer a configuration variable of some kind to specify the DSN. You must enter the something.db name for this purpose. You cannot use localhost or leave it blank, or your program will not work, and you'll get this error message.