How do I add an FTP Account for my additional domain?

Follow these steps:

Login to your control panel
Under the FTP Tab (near bottom right section)
Click "FTP Accounts"
Click "Add Account"
For the login field, enter the sub-domain name you used earlier.
Enter a password

Click "Create"

The login name will become the created directory. If you entered "joe", a directory called "joe" will be created under your "www/public_html" directory. If you already have a directory called "joe" then a new directory will not be created.

To login to your FTP account use the following settings in your FTP software.

hostname: yourdomain.com or your IP address
login: username@yourdomain.com (joe@yourdomain.com)
password: enter_your_password

Users will only have access to the directory you created. If you created a login name called "joe". Then once logged in, joe can only access the "joe" directory and anything below that, e.g. yourdomain.com/joe/test/. Joe will not be able to access any directory higher than his directory.