It works fine on the older/standard Magento cart. Its a simple switch of the POST URL address in the admin section, that’s it. If you have problems, call NMI’s support # listed in the comment above.
FYI, to understand the API responses, if you are using authorize.net emulator, you actually need to download the API Integration manual from authorize.net (even though transactions are sent to NMI, the response codes are the same). There are a few quirks but vast majority of API responses match.
Here is their “AIM” API to reference: http://developer.authorize.net/api/aim/
Lastly, the new HOSTED “Go” version of Magento seems to not allow the merchant to edit the POST URL and so the new hosted/Go version may not work with the NMI emulator from what I’ve seen so far (unless someone knows a way to edit the POST URL?)