Right - on the site it says
1 Create an AOL screen name (this will be your AIM log in)
If you already have a screen name then skip straight to step 2
2 Download the latest version of AIM
3 Start chatting to your friends online in real time
To sign on:
1. On the AIM Express sign-on screen, enter your screen name in the Screen Name field.
2. Enter your password in the Password field.
3. Click Sign On.
as I read it .... if you already have a screen name (which by the sound of it you do)
all you need to do is sign in using your details
if this has worked on other 'puters .....