Looks like that sales page is missing the seller's View Cart button code, which loads our javascript that displays the cart as an overlay within the page and includes the function necessary for multiAdd to work. If you don't want a visible View Cart button, you can just paste the script portion of the View Cart code:
<script type="text/javascript">
function EJEJC_lc(th) { return false; };
</script>
<script src="https://www.e-junkie.com/ecom/box.js" type="text/javascript"></script>
We'd advise putting the View Cart code on every sales page, so the overlay-style cart can work there. Whenever the overlay-style cart cannot work for any reason in a given page (such as missing the View Cart script), regular (non-multiAdd) Cart buttons on that page should still work in "fallback" mode by displaying the cart in a popup window/tab. However, items added to the overlay-style cart will not appear in the fallback/popup cart, nor vice-versa, and since the overlay cart is necessary for multiAdd, you'll want the overlay working on every page.