Authorize.net payment not shown in Magento checkout page

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".