To configure Auctria to use an Authorize.Net account you need to enter your API credentials from the Authorize.Net dashboard.
Click on Credit Cards then Authorize.Net Connection
In the Authorize.Net Credentials section you need to provide the API login id and transaction key. You can find these details in your Authorize.Net admin page at https://account.authorize.net Click on Settings and then API Credentials & Keys
This page will show your API Login Id (a sequence of a dozen or so numbers and letters) and also let you create a new transaction key, both of which should be copied and pasted onto your site.
When you save the credentials on Auctria you can then click the Sync Public Key button. Assuming that the credentials are correct this will retrieve the public key associated with your account. You will also see details about your merchant processor shown in the top section.
If there is a problem with the credentials you will see an error instead, in this case please double check that they were copied and pasted correctly from Authorize.Net
Authorize.Net & Test Mode
Authorize.Net does not have as robust support for test mode as Stripe.com has. You can not run test mode transactions through a live account. When you put an auction into test mode (see Test & Live Modes) the charges by default will be run through Auctria's sandbox account: note you should only use test card numbers in this case.
You can if you prefer open your own Authorize.Net sandbox account and configure it here but there is no real benefit to this other than being able to see the test transaction reflected in your own sandbox account.