It looks like the database connection is being refused; please validate the correct MySQL database user, host, and password are correct. Some control panels have their user accounts as [site username]_[created database user] (ie siteuser_username), please validate that wherever the username is displayed that it is displayed EXACTLY as you're entering. If you are copying and pasting you'll want to make sure that you're not copying any " " blank characters (which tends to happen when copying from an email or a web page). You'll also want to make sure that the created MySQL user has been granted sufficient access to the new database that you've created.
Some control panels do not assign the same password to the MySQL database user as the FTP password; try going into your web hosts control panel and change the database password to what you think it already is.
The table prefix is simply what you'd like the table names prefixed with; if you have multiple installs in the same database you would set the prefix on install B as a different one from install A so that the databases can run along side eachother. The default is fine, and there is no validation necessary for this field.
I have just validated that the latest copy of the v7.6.1 install archive from the member area is capable of running a new install without any problems.
