I am slightly confused here, are you asking for how to add a parked domain to your account?
If so, then you need to log into cPanel and click the "Parked Domains" icon. From there, you can add your parked domain to your account.
If that isn't working due to errors about domains already being configured and such, or if that was not your question, please post back with a clearer explanation of what you are trying to do or what domain you are trying to park that is giving you errors.