Shopping Cart Mode allows customers to view products, add them to the cart and place the order for them. To enable "Add to Cart" button for a product, you need to check all of the following conditions:
Make sure that Catalog Mode is turned off (Go to EShop -> System -> Configuration -> Option tab, choose No for Catalog Mode).
Make sure that product price is showing on the front-end side (Go to EShop -> System -> Configuration -> Option tab, check for Display Price option).
Go to edit product to make sure that you don't enable "Call for Price" feature for the product.