Login to Plesk Control Panel at https://plesk.colony1.net:8443/ 

Once logged in go to 'Domains'
Click on 'YourDomain'
Click FTP Accounts (under Files Tab)
Add New FTP Account

FTP Account Name:
Home Directory: /httpdocs/  (leave as root '/httpdocs' or choose any subdirectory)
Password:  (Requirements are at least 3 upper, 3 numbers, 3 lower, (eg. GRT483xtw))
Read permission:  (view and download permission)
Write permission:  (create, rename, and delete permission)
Click, OK

Now if you FTP to Host: 'YourDomain' with the user/pass from above it should work.

Need a FTP Client Tool? Try FileZilla: http://filezilla-project.org/download.php