Authorize.net - Login ID and Transaction Key

When you're logged in to Authorize.net, go to Account > Account and API Settings - then API Credentials and Keys:

If the Login ID has already been created, you can see it on the API Credentials and Keys page. The Transaction Key is not retrievable on the site, so if you don't have it written down, you'll need to generate a new key. Once created, this will only appear in a pop-up window.