Hi Mike,
Are you using a live or test account to test your transactions?
Regarding the decline, are there any errors being logged to your DNN Event Viewer at the time of the transaction?
Also not sure if you've seen it yet but we there are instructions for the Moneris gatewway in our Gateway Guide at the following link -
http://www.smith-consulting.com/Porta...
Let me know if you have any questions.
Thanks!