Chris,
Have you used ACM with all its features active (not the demo) before? It should only ask you for the key once, unless you remove the program at some point. When you open ACM there's a prompt that says "this product is not registered" which means it's a demo, hit OK and then ACM opens up with (I think) FS2004 as the default aircraft folder. Click on Options and select "Activate", that's when I entered my key#. Now it's registered. Unless your ACM is different from mine (v2.5), that's all there is to it. It never asked me to register as a user, with name, e-mail address, etc.