The domain is already added to your account. You can easily see it in the client area: it’s clearly shown in the domains list on your account detail page, as well as the bi domains list that displays all domains you have across all your accounts.
So why can’t you find it in the Addon Domains list? Simple: it’s not an addon domain, it’s a parked domain. Those are two different lists in the control panel.
And that’s confusing, I know. That’s why the client area doesn’t separate those lists. I recommend to use the client area to manage your account instead of the control panel, as many features there are more powerful and easier to use.