If you would like to display your subproducts as a list of checkboxes there are two areas in the product template that you would need to edit. The DEFINE SUBPRODUCT section of the product template and the DEFINE PRODUCT section of the product template.
Replace the DEFINE SUBPRODUCT section in your template with the following.
After you have setup the code for the DEFINE SUBPRODUCT section, you would need to include the LOOP SUBPRODUCT tags with the DEFINE PRODUCT section of your template where you would like the subproduct checkbox list to appear. The LOOP SUBPRODUCT tags would look similar to the following. Note that you would still need to include the add to cart form (see product add to cart) so that the products who's checkboxes are checked get added to the cart. You do not need to include the <input type="hidden" name="itemnum" value="[-- PRODUCT.RecordNumber --]"> which is part of the add to cart form.