The database name has to be set in the source code for postgresql, which is not very user friendly. The default database is "template1" and if this database does not exist, the login fails.
You seem to have CSS turned off.
Please don't fill out this field.
Is there some database which always exist? E.g. "postgres"?
You can use ?db=yourdb in URL to use this database directly. I don't want adding the field to login form because it is already presented in the left column after logging it and it should be <select> instead of <input> if possible.
You can also create extension with loginForm() method: http://www.adminer.org/en/extension/