I tried to set up a control panel for one of my domains, but I cannot log into the control panel for that account using the username and password I set up.
Under the "domain user" section, don't forget to check the "Allow domain user access" checkbox - once you check this you should be able to login to their control panel.
Login Name: theirdomain.com
Password: the password you entered
IMPORTANT: Be sure and tell them their login is theirdomain.com, not their ftp login (this has confused several people)