Can you explain what you mean when you say you're trying to "find a new screen name?" If you are creating a brand-new screen name, you shouldn't get these messages.
You can setup a new screen name
here. Screen names must contain from 3-16 letters and numbers, and they must begin with a letter. You cannot use spaces when you create a screen name.
Passwords must contain 4-16 letters or numbers.
The messages you describe above would be if you're trying to login to an existing screen name, but have the wrong information. If you've lost your password, you can
request an email reminder. This only works if the email address you provided when you initially setup your AIM screen name is still valid. If it's not, you are out of luck.