Some configuration to your account must be done at Authorize.Net's Web site (Authorize.Net will have supplied you with access details). This is referred to as Authorize.Net's Merchant Management System. Login at:

https://secure.authorize.net

1. Go to 'Settings and Profile' and select 'Relay Response'. Change this to the callback URL of your site, i.e. http://www.sitename.xyz.com/callback.aspx?g=authorizenetsim.
2. Click on 'Receipt page' and 'receipt method'. This sets the page on your site that a user is returned to after finishing a transaction (normally should be http://www.sitename.xyz.com/checkoutcomplete.aspx)
3. Set the receipt method to 'Link', and text to "Click here to return to the store", or something similar.
4. Go to 'Settings and Profile', under 'Security', select 'obtain transaction key'. Enter your secret answer to obtain your key. This key must be entered in the TransactionKey setting (see 7. above).