I know that I have had problems downloading the update for the Carla theme. So, maybe, the repo is loaded up right (on the developer end)?
All I can suggest to do (which is all I'm doing) is to keep trying to get them. Oh, and make sure they are for the iPad. Carla started out as an iPhone theme, so there will be a lot of "stuff" about Carla - on Cydia - that are only for the iPhone. That may also be a reason why you're getting the "forbidden" notice...
Marilyn
EDITED TO ADD: Forgot to say this part - you can't just get a "module" for the theme, even when you've paid for the main theme. The theme must be already downloaded to your iPad or part of the download action you're doing. So, maybe you're getting the error because you don't have the main theme loaded? You don't have to activate the whole theme, just the part you want. But - it must be on the iPad (it's the developer's way of preventing pele from getting the h]theme parts without buying the main theme).