If you are getting the invalid XML error message, try testing in Firefox instead of IE, FireFox will still show the XML even if it isnt formatted correctly, IE wont.
You can get the full request and Full response bindings by going to the bindings tab, click the plus button. Select eCart -> Cehcout -> Local checkout Response.
From the list of gateways, select Authorize.net AIM.
On the bindings panel will be a new local checkout response group. add the full request and full response bindings to the page.