It’s great to use One Page Checkout, but it’s broken. We can’t continue after Shipping step.

We can fix the base template

app/design/frontend/base/**default/template/checkout/onepage/payment.phtml**

Or our custom template (e.g. magento-foundation theme)

app/design/frontend/magento-foundation/**default/template/checkout/onepage/payment.phtml**

There we need to add an id attribute to the following fieldset html element (this loads the payment methods):

<fieldset>
    <?php echo $this->getChildHtml('methods') ?>
</fieldset>

Change it to the following code

<fieldset id="checkout-payment-method-load">
    <?php echo $this->getChildHtml('methods') ?>
</fieldset>

We have added this

id="checkout-payment-method-load"

Source: Can’t checkout on CE 1.8 - Magento Forum