When installing Magento, we may get such error "The URL is not accessible".
In some earlier versions of Magento, we can fix this problem by change code of app/code/core/Mage/Install/Model/Installer/Config.php
$client = new Varien_Http_Client($url . $prefix);
$client = new Varien_Http_Client($url . 'index.php/' . $prefix);
Will solve the problem. It's because that the URL rewriting doesn't work for some reasons. (Magento uses .htaccess files to add URL rewriting rules)
But in Magento 1.9, checking the "Skip the base url validation" option will solve this problem
There are mainly two reasons causing that Authorize.net payment method not showing in Magento checkout page
The Mage_Authorizenet and Mage_Payment module is not enabled
In Admin page, go to System->Configuration menu. In Advanced->Advanced section at left panel, enable the "Mage_Authorizenet" and "Mage_Payment" module.
The magento currency is not same as Authorize.net currency
Assume you are using US Dollar as your currency.
In Admin page go to System->Configuration menu. In General->Currency Setup section, set the Base Currency and Default Display Currency to "US Dollar". Then go to Sales->Payment Methods section, under Authorize.net (or Direct Post) tab, change the Accepted Currency to "US Dollar".