To get started you are going to have to implement a DataAssist Search server behavior onto your add to cart page. You can think of this as your search page, the search will be performed on the values that you have listed with the checkboxes.
You have a recordset that selects all of the records that the user can select with the checkboxes. Once this recordset has been crafted you add the DataAssist Search server behavior to the page. When you configure this you will setup the search criteria to look for values that match the checkbox fields you have. You must ensure that your checkbox fields have the values set the same as the values you will be searching on. For example if you have the record id as the value of the checkboxes you will need to be searching the id column in your recordset where the checkbox value matches that column.
Once you have got this working you will be ready to use the add to cart from recordset server behavior to add all the user selected items to the cart. If you have any problems getting the DataAssist Search server behavior implemented make sure to take a look at the DataAssist support page for additional resources to help you get this in place.