The Auth.net AIM payment method has been deprecated (auth.net themselves have deprecated the AIM intergration so we have deprecated the payment method that uses it) so AIM no longer shows up in core.
However, the replacement for it is Auth.net Accept which you have access to.
On your account page download th Auth.net Accept payment method, install it and activate the payment method.
It’ll then show up in The EE payment methods for you to set up. I’ll get the quick start guide updated to reflect this.