How to configure Authorize.Net?

You need to set the following in Settings -> Receipt Page -> Receipt Method in your Auth.Net account:

Default Receipt Link URL: http://www.yoursite.com/system/payment/callback/authnet_sim/

Receipt Method: POST

Receipt Link Text: SOMETHING_NOT_EMPTY

Also you need set the value of Authorize.NET account -> Settings -> MD5 Hash equal to the MD5-Hash value defined in the settings of "Authorize.Net SIM" gateway in iAuto Admin Panel. This value is "first" by default.