The passwords entered into the Credential Manager can only be used in password fields inside certain actions. For example, if you add an "FTP: Logon" action, you'll see a key icon to the right of the password field (see attached screenshot). If you click on this key, it will show a list of available passwords to select.
To enter a password onto a website, you would need to use an "Interactivity: Set Text" or "Web Browser: Set Value" action which neither have the option to use the Credential Manager passwords. In this case, you can save the password as a Constant and reference the Constant in the action.