This error message means that you have already registered with Sdui with this code. The activation code can only be used once to register in Sdui and becomes invalid when the registration is completed. Your activation code is not a password.
When you registered, you provided a user name or an email address and your password. These are your login details for Sdui.
If you have forgotten your password, you will have to reset it. On the login page, go to "Forgot password?" at the bottom.