The DSN, or hostname, for connecting to MySQL is the name you gave when you created it, followed by the extension .db. The full connection information can be found in the email sent to you when you created your database. Never use localhost as part of your database connection information.
If you need to find your DSN and no longer have this email, visit the MySQL panel. The DSN for your process appears in the first column. Clicking on the DSN name will take you to a page with more information about your process, including your default admin username.
DSN stands for Database Server Name and is sometimes also referred to as hostname or MySQL Server Name or even just server name. This is different from Database Name, which is something you choose separately for each program that uses your database (and usually create from our interface, the command line, or a tool like phpMyAdmin).