Aaron -
Please go through the steps here again:
http://www.methodintegration.com/cs/blogs/methodblog/archive/2011/02/09/intuit-payment-solutions-intuit-merchant-services.aspx
Make sure you choose "No" for step 12.
We've seen this before when someone initially goes through the steps and chooses Yes for step 12. It works for a while, and then the authorization expires so it stops working.
Also, have you ever been able to get it to work? Or was it working and then suddenly died for you?
HTHs,
Paul