Im new at this too... but i think u are missing the include file...
go to...
windows -> Bindings panel -> click the + sign (at the top left corner)
ecart -> checkout -> local checkout response ->
choose your payment gateway (authorize.net)
after that you will see a Local Checkout Response (Aunthorize.net) option in the source col.
click the + and drag the first 4 things into the the page...
they should look something like this
reason code: <?php echo(AuthNet_Result("Response Code")) ?><br />
resaon subcode: <?php echo(AuthNet_Result("Response Subcode")) ?><br />
resaon code: <?php echo(AuthNet_Result("Response Reason Code")) ?><br />
reason text: <?php echo(AuthNet_Result("Response Reason Text")) ?><br />