This page is where you can place your orders. If you're logged in to your account *and* the market is open, you will see quantity box an "Add to Cart" icon next to each item. If you just want one, click on the icon to add it to your cart. If you want more, you can either change the quantity and then click, or click the number of times you want.
Once you've finished shopping, click the checkout link in your shopping cart (located at the upper left side of this page, below this help area).