The add to cart buttons can be used to add static products to the cart. eCart will build the rest of the cart interface that you are looking for so that it is dynamic.
The page that you apply the cart display will be the first dynamic page that will keep which items you have added to the cart. From there, the checkout will forward you to the checkout page where the user enters their information or chooses the quick checkout if you decided to include it. Next the user is forwarded to a confirm page and if the user confirms the information it is sent to the payment gateway and depending on the results, a transaction success or failure page is the last step.
You said the cart wasn't being added to the page, were you getting any errors when adding to the page? What steps have you taken in the process to add the cart to the page?