In the Downloads Center for eCart there is a section titled eCommerce Recipes. The eCommerce recipe PDF File contains a recipe for quantity discounts which will explain how this is done.
For offering a free item, you will have to write custom code for this.
You really shouldn't add the item to the cart itself for a number of reasons. If the cart is updated, you may want to remove it. the customer would also be able to edit the quantity for a couple of examples.
You could write an If statement to show it in the cart based on the order total.
Then add an Insert record behavior to the checkout process to store a record in the order details table for the free item. you would need to hand code the if statement for the Insert record behavior to happen only if the order total met your criteria.