Magento – Fatal Error Mediafallback.php on line 61

The error simply says, Magento couldn't find Zend_Log class inside your application. By default, this class resides in \lib\Zend\Log.php¹. So the first thing that you want to make sure is, this class exist in your magento instance.

But the bigger mistake that I can see here is, there is a Core Hack, which you should never do in Magento. If you want to extend Magento functionality, you should either do a rewrite or if possible, use an event handler.

In this case, default Mage_Checkout_Block_Onepage class do not have a method getAddressesHtmlSelect(), but it's parent class (Mage_Checkout_Block_Onepage_Abstract) has one. So I doubt someone is trying to modify it's parent class method behaviour by directly putting the method inside Mage_Checkout_Block_Onepage. This is completely wrong.

I am enforcing the point. "In order to do this, you should either use any event handler or rewrite the class Mage_Checkout_Block_Onepage".

Note : We can't see any Zend_Log class usage in the provided code. So further help is not possible, if you didn't provide more details.

¹ - Zend Log class

That means, the source model for these form fields could not be loaded. Check if the following files are present and readable:

• app/code/core/Mage/ConfigurableSwatches/Model/System/Config/Source/Catalog/Product/Configattribute.php
• app/code/core/Mage/ConfigurableSwatches/Model/System/Config/Source/Catalog/Product/Configattribute/Select.php