If you used 'one-click-install' to install WordPress and did not modify the default path, then the files should be in the root directory of whatever domain you chose to install it to. For example, if you installed to example.com, the WordPress files will be located in a directory called example.com
The password you use to log-in via FTP may or may not be the same as the password you use to access the panel.
To log-in via FTP you need to use the username and password of the user that controls that particular domain. To find the password for a particular user go to Users -> Manage Users in the panel and click Edit for the appropriate user. The password will be shown at the bottom of the page.
Note: To download your 'whole blog' you will also need to export the database used by your blog. This can be done using PHPMyAdmin. You can log-in to PHPMyAdmin via the panel at Goodies -> Manage MySQL and clicking on the phpMyAdmin link for the relevant database.
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