You'll need to edit the line that says this:
define('DIR_FS_DOWNLOAD', DIR_FS_CATALOG . 'download/');
to look something like this:
define('DIR_FS_DOWNLOAD', '/home/my_user_name/download/');
Specifically, you need to indicate the exact complete path for the webserver's filesystem that points to your download folder. You could use your DIR_FS_CATALOG entry in your configure.php file as a reference, and make appropriate adjustments. Your FTP program and/or your webserver's hosting control panel should be able to help you understand the complete path.