Creating the cart object should only create a new set of cart pages in the WA_eCart folder. Once the eCart object has been created you should be able to edit the various parts of the cart object from the eCart object panel. Do you have this panel open? What happens when you click on the edit? Can you see your cart object in the list of available carts if you click on it?
I have seen some problems with the eCart in general when it is used in the pre-release version of Dreamweaver CS3. If you have build 3453 it will not work for you, Dreamweaver needs to be build 3481 or higher for it to work properly. You can check the build number by going to help > about and clicking on the scrolling text.