1. Use Filezilla or a similar program
Download filezilla at http://sourceforge.net/project/showfiles.php?group_id=21558 using the latest release.

2. Go to File -> Site Manager, click on "New Site"

FTP to:
Host: your-domain-name.com
Port: 8021 IMPORTANT
Logon Type: Normal
User: your username
Password: your password (call if you don't have it)
Default Remote Directory: IMPORTANT
/clients/your-domain-name

Example: Host: drop-in-the-ocean.org
Port: 8021
Logon Type: Normal
User: your username
Password: your password
Default Remote Directory: /clients/drop-in-the-ocean

3. Alternatively, try:

ftp://username:password@www.your-domain-name.com:8021/clients/your-domain-name

For example, if your username was "david" and your password was "123" ftp://david:123@www.drop-in-the-ocean.org:8021/clients/drop-in-the-ocean