CARS registration takes place within the AAPEX registration site. If you follow this link it will take you to the CARS landing page in the site. When you click to register you will be taken to AAPEX registration. You must be registered for AAPEX to register for CARS and TTF.
Once you get to the list of add on programs at the end of registration you can click the heading at the top of the page for Thursday (TTF) and Friday (CARS) to more easily find the check boxes. The session IDs are TTF and ASA. Click those boxes and they will be added to your shopping cart.
If you have already registered for AAPEX/SEMA you can still add the sessions by visiting your dash board.