I'm not certain about the procedure in Carrara, but do know how to do it in RDS (and most RDS tricks translate to C). If you're working in the mac environment, render a Quicktime; the alpha channel just automatically comes along as a pre-multiplied with whatever color you have selected for your background. If your in the PC environment, you need to render your animation as a sequenced photoshop file. When you're importing the image into AE, be sure that you check the "sequenced images" tick box in the bottom of the import menu. (That little tick box caused me days of grief....) Hope this helps!