No Checkout Button

Your cart is not displaying anywhere on that page, which seems to be a problem with this plugin: it does not always display the cart on single product pages. To fix this, check in the Settings > Store > Presentation, where it indicates the Shopping Cart settings. If “Page” isn’t working, try a widget, if that’s compatible with your theme. I use “Manual,” then insert the shopping cart code: <?php echo wpsc_shopping_cart(); ?> into the ecommerce template file wpsc-single_product.php.

As for where to place that code, it depends upon where you want the cart to show up. I have styled my cart in CSS to be absolutely positioned, so it doesn’t matter where I put the code.